She lets him live.
Looking back, Bloom decides she allows this to happen for two reasons. One is the fact that Baltor - despite being a power hungry, poorly dressed sorcerer with a mile-long hit list and a black hole where his heart should be - is human. She hated Lord Darkar, and has no problem admitting so, even now. And Bloom knows that’s because his humanity had been lost long ago. She could've spent years trying to coax his original self back from under that barren shell of the Lord of Shadowhaunt, but it would've never done her any good because the human in him was dead. So erasing him from existence had been easy (the kidnapping and brainwashing her into an evil henchman also helped). Baltor...he obviously had problems, but he looked like a human and spoke like a human (with a slight case of demon possession, but she was sure a few spells could fix that), which to Bloom made him human. She's never taken a life before, and didn't plan to start now.
The other reason is much more selfish, if she actually stops and thinks about it. The longer the battle with Baltor went on, the more obvious it became he was just as clueless about her parents as Bloom was. This could be good; it meant they could still be alive. Hell, her whole planet could still be alive under the layers of ice and rock knowing the Ancestresses and their tricks. And she wants them to be alive. It's a desperate feeling that twists her insides at night when she tries to sleep.
But more prominent than that desperation is the creeping feeling she's wrong. That the months - years - of her life dedicated to resurrecting a dead planet will ultimately fail, because that seems to be the only thing she can do in reference to Sparx. And not only would she be hailed as the failure of her home world, but also the last person left from it.
It’s the latter of those two thoughts that scares her the most. Her mother always said she had a fear of being alone and tried to equate it as some deep-rooted result of being adopted. Bloom doesn’t really care about the reason. What she does know is there’s exactly one other person left with a connection to her home world and her powers, even if he was insane. And if she did kill him only to fail later on resurrecting Sparx, she would have no one to blame for her loneliness but herself.
So she improvises.
The party celebrating Baltor's 'death' is full of dancing, kissing, and drinking. Bloom does her best to act both happy and humble about her not-really-real-victory. In the end she doesn't need to act very hard, because everyone's so busy celebrating they don't notice how nervous she is.
Nor does anyone notice her sneaking off a mere hour and a half into the party, walking through the trees at the edge of the clearing and not coming back.
Icy, Darcy, and Stormy had given up the location of their last hideout when the Light Rock guards showed up. It had quickly been scanned and cleared out of all the stolen spell books and artifacts by the time Bloom got there. Since no smart villain returned to a hideout once it was discovered, and Baltor is the least-idiotic villain she's ever faced, Bloom figures she can keep him there for the time beng (Step 1 of Improvising: Use a combination of invisibility and levitation spells to bring said unconscious villain to their former hideout).
Her fairy dust hadn't destroyed Baltor; it could only get rid of dark spells. But luckily that meant it could destroy the nasty demon the Ancestrsses had infected him with. And Bloom figures the few hours she spent getting ready for and attending the party are be enough time for the spell to do its work. And for her to formulate a plan.
Both her assumptions are correct when she enters the hollow cavern and finds Baltor very much awake, struggling at the fire chains (Step 2 of Improvising: Lock villain up with Dragon Fire chains) keeping him pinned to the stone chair. He looks worse for wear, the bottom of his jacket singed black and blood in his hair. A bruise is starting to show on his collarbone, green and purple mixing together and contrasting against his pale skin. Bloom's never seen him look this bad in the entire year they’ve fought.
But at least he's still alive.
Baltor stops struggling when she comes into view. Bloom waits for him to say something or start yelling, but the cavern is silent aside from the occasional drop of water hitting the floor from a crack in the ceiling. He looks at her like he wants the gods above to come down and smite her, and it makes Bloom feel powerful. That and the fact she sees his hands twitch in the chains, trying to summon magic that won't come to his fingers.
"I assume you've heard of the Nullis Lux spell." Bloom finally speaks. "It originated in one of the books you stole."
The answer to her question is obviously a yes, because for a moment Bloom sees fear in his eyes. "You used the Nullis Lux spell on me?" Baltor finally asks. His voice is hoarse, and Bloom can see blood on some of his teeth.
Baltor rolls his eyes, and spits a mouthful of blood out on the ground (though not in Bloom's general direction, which makes her feel a bit better). "That's impossible. The spell takes away the powers of the weaker being; there's no way you could've beaten me so easily."
Bloom has to smirk at his still-inflated ego. "You should thank Professor Avalon. He taught me fairy dust could remove dark entities, even though you tried to tell me otherwise. After that was gone…well, stripping your personal powers weren't that much of a problem." (Step 3 of Improvising: Use a magic spell to get rid of your enemy's power) Baltor coughs and spits another mouthful of blood out in response, and Bloom involuntarily winces. "How do you feel?"
He glares at her. "Like shit, no thanks to you."
"You're welcome for sparing your life."
"Please, I don't want your pity."
"I didn't help you because I pity you." Bloom says, sitting opposite of him on the cavern floor. He looks towards the ceiling instead of at her face. "You've killed hundreds of people and caused millions of dollars in damage all because you couldn't win a fight. Your ego could cover my entire hometown and then some; I'm way past pity."
"Then educate me, Your Highness: what caused your change of heart?"
Bloom doesn't answer his question. "I'm going to go out on a limb and say you don't want to be sent back to the Omega dimension for all eternity. And lucky for you, there's an alternative solution."
"Lucky me," he quips. "What deserted dungeon do you plan to imprison me in?"
His head snaps down to properly look her in the eyes; the first time since she's entered. "You must be joking."
"Nope. Earth is a magic-less world with enough government to keep you in check. And who knows; maybe if you don't try to overtake it, I could give you your powers back." Even as she says the words, she knows it isn't that good of a sell, especially for someone like Baltor. But she had hoped the idea of maybe getting his powers back would help sweeten things.
No such luck. "So you're keeping me alive as a pet project? A convict you can reform into a respectable member of society?"
"You're not a pet project; once I send you to Earth you can walk into the White House and proclaim yourself leader of the free world for all I care. But I'm warning you; you won't get very far without your powers."
Baltor glares at her. "Sending me to Earth with a lack of magic still sounds like a pet project. Since this has turned into some sort of honesty hour, you must humor me; why didn't you just kill me?"
"Do you want to die?" He huffs and doesn't meet her eye, which is the closest thing to a yes Bloom’s going to get. "So again, you're welcome."
"…You've never killed before, have you?"
She blinks in surprise. Baltor's gaze is still on the wall, but he keeps speaking. "It’s for the best, really. You don't want to know what it's like. Standing over someone and watching the life fade from their eyes. It doesn't even matter if you meant to kill; you get this feeling that washes over you that you can't place, and it stays with you for weeks on end. And by the time you do work out what it is, you've killed too many people to care."
The speech he gives is unprompted, but Bloom would be lying if she said it hadn't been in the back of her mind. "What's the feeling?" she asks when he still won't look at her.
"Emptiness." There's a beat, and he finally looks back at her. "No one would've objected to you killing me."
He gives a humorless laugh. "Do they know you didn't kill me?"
She debates lying to him, but knows if she does now things will just get worse down the line. "No. They don't."
"And why is that?"
"Because you're a coward who fears death…and I'm a coward that fears loneliness."
To her surprise, he smiles. Not a full smile – there's still a hint of a smirk behind it, but for the time being, she'll take it. "We're in each others hands, then."