Minerva doesn’t need the killing curse. Point of fact, she is one of the only members of the Order who didn’t sully her magic that way in any of the wars she’s fought in.
That isn’t to say she hasn’t killed.
She also doesn’t trust that Fawkes won’t flame in to defend his master from a killing curse. Instead, when Minerva apparates into her home—still standing—and kisses her wife—still breathing,-- and puts on her robes for the day, she’s not prepared for a duel.
She sits through the teachers’ orientation meeting, fury churning like a cold sea behind her occlumency shields, and when Albus Percival Wulfric Brain Dumbledore finishes his lemon-flavored tea, he falls over dead at the table.
Minerva doesn’t scream. She’s always kept a level head in a crisis and that serves her well today. She jumps to her feet, already shouting for Sinestra to get the aurors—her office is closest.
Pomfrey is already performing diagnostic spells, blood paling out of her expression.
“What is it?” Minerva asks tersely, still unconvinced that the cockroach won’t survive this too somehow.
“I—I don’t believe it. The strongest pain relief potion I’ve ever seen and…”
“Some type of poison. I’ve never seen anything like it. It’s almost like it dissolved his insides on contact.”
Minerva puts a hand over her mouth but it’s primarily to hide her smile. She can’t laugh now, can’t ruin this; she absolutely mustn’t cackle like a mad thing so she skips right over that stage and cries with relief. Pomona puts a shaky hand on her shoulder and looks to the medwitch.
“Maybe—St. Mungos? What if we floo him immediately—”
“No.” Pomfrey says grimly. “No, I’m afraid—it’s much too late for that.”
“What do you mean?”
“I mean the poison was so strong it cut through his esophagus and melted his heart and lungs—” Pomfrey cuts herself off with a sharp inhale.
“So he’s dead then.” A quiet voice says and at first she thinks it must be Severus, shell-shocked in the corner, but Severus is already moving forward to try to identify the poison from the tea cup. Belatedly Minerva realizes it was her own voice.
Her rear-end hits the chair she’d shot out of in alarm as her knees give out.
“On, Minerva—wait. What on earth?”
Minerva can only sit, holding her tongue as one-by-one the loyalty compulsions and god knows what else snap with the true death of their castor.
“Merlin’s beard!” Filius topples over in his own seat.
Later that day—after a long, long process and thorough interviews for each of them—Minerva sits at the headmistress’ desk and unfolds the heavy scroll before her.
Did you kill Albus Dumbledore? They’d asked each of them, and under Veritaserum Minerva had answered: “No.”
The bastard had killed himself as far as she was concerned; by drinking his death with his own hand and by the decades of evil actions and flat-out black magic he’d committed in the name of his precious greater good.
He brought that shite on himself.
Did you put poison in his cup? They tried to cover their bases but the war had—just—ended and Death Eaters were running rampant trying to find hide or hair of their fallen master, and the tax on the aurors showed. Not ‘Did you put anything in his cup?’ not ‘Do you know how poison got into his cup?’ Not any sort of question that would damn her.
Not ‘Did you travel back in time with the sole purpose of murdering him?’
No, she said, and she could have been compelled to say more but parselmagic occlumency shields laughed at average strength veritaserum. Basilisk venom was hardly a poison.
She was the deputy headmistress; she was well known to be Albus Dumbledore’s right hand witch and after the cursory questioning she was let go, with words of comfort and reassurance.
With Aberforth all but having disowned the man, it fell to her to set his affairs in order. He hadn’t even had a portrait commissioned. The bastard truly thought he’d die on his own terms or not at all.
She sat at her desk, realizing she’d truly seen the last of the man who’d ruined everything.
Their entire world.
To Minerva Isobel McGonagall nee Ross I do bequeath every transferable duty…
She is, per the will, the temporary Supreme Mugwump until one is chosen at the emergency session and her first order of business will be to ensure every prisoner in Azkaban got sent there with a fair trial.
There’s a lot of work to be done cleaning up Dumbledore’s messes, but Minerva has sacrificed everything to come back here and do it. She’s hardly going to balk now.
I only wish I could have killed him twice. Or six or seven times, really.
There was work to be done, but she stroked her finger over the ink that declared her magical guardian to ten separate children. She’d always wanted one and here these were, alone and waiting for her, muggleborns and orphans who had no one else.
Minerva McGonagall sat, victorious in her vengeance, Headmistress of her school and finally, properly magical guardian to, among others, one Harry James Potter.