Peter's always been a voyeur. Sight is more intimate to him than touch is; he doesn't know why, but it just is. He's always lived his life on the outside looking in, even as a youth, even before he discovered his, ah, latent villainy. (No longer latent, thankfully. Self-denial is such a bore.)
As a teenager, Peter amused himself by closely following the lives and private affairs of several members of his pack, and interfering with or aiding their various romantic ventures. He fancied himself a bit of a capricious Cupid, then, malicious and benevolent by turns, building bridges one minute and burning them the next. Of course, his targets never quite deduced the role he played, and that amused him most of all. Peter was made to operate in the shadows, from the shadows. He warmed himself with the light of others, for he himself was too cold.
Sometimes, when his subtle conspiracies united two eager lovers, he found a place far enough to not be scented, and watched them. He saw the desperate meeting of mouths, tongues slick and gleaming, the hands scrabbling across clothing and quickly-stripped skin, the parting thighs, the rutting hips. He jacked himself off, coming harder than he did whenever his girlfriend of the month touched him. Possibly, he had a problem. No, he knew he had a problem. At least, the vast majority of society would call it a problem.
But the definition of what is and isn't a problem is always subjective, isn't it? Not one for lying to himself - lying was something he reserved for others - Peter simply accepted that watching worked better for him than touching did, and got on with his life. And if his primary form of entertainment was spying on folks he may or may not have manipulated into doing the nasty, so what? The fact that most of them were pack members he was related to by blood didn't bother him. It wasn't like he wanted them. He just wanted what they did.
Derek's affair with Paige was a... tragic miscalculation, because Peter hadn't exactly planned for her to die the in the manner she did. Still, it was worth it for the agony writ large across Derek's face in the months that followed, transparent and stupidly obvious, and Peter felt a strange sense of satisfaction at that, because Derek and Paige had been a little too sweet, and Paige hadn't been worthy of joining the Hale pack, anyway. Peter wasn't the Alpha - not yet - but he would ensure that one day, he would be, and that he would only permit the best of the best to join his pack.
Now, more than a decade and two resurrections later, Peter is quietly cultivating a pack worthy of him, allowing them to 'find' each other as if without his interference, starting with Scott finding Derek and Derek finding Stiles. Sadly, Derek's stubbornly resistant to turning Stiles, despite Stiles's suitability, because Derek wants Stiles and has learned from Paige never to turn the one he wants, and from Kate never to trust the one he wants.
Peter spares a moment of uncharacteristic regret for the Paige Syndrome, but is determined that Derek will turn Stiles, regardless. It's the only way Stiles will join the pack. Stiles turned Peter down when he offered, but he wouldn't turn Derek down, because he desires Derek just as blatantly as Derek desires him. It's ridiculously plain to see. And Peter should know, because he sees everything. He makes it his business to see everything. Ever since he missed Kate Argent's plot, he's been more watchful of others than ever. He'll never forgive himself if - if the pack he's building now is -
It won't happen again.
All Peter has to do is engineer Derek's leadership of his pack without seeming to do so, ensuring that Derek makes the right decisions at the right time, ending with Scott in his pack. And then, Stiles will be soon to follow, because a) Stiles follows Scott everywhere, and b) Stiles does want to be a werewolf, and will say yes to the bite eventually, especially if the offer is made by the thick-browed dolt he's in love with. If Derek balks, Peter will play Cupid once again, locking Derek and Stiles in a goddamn basement together if need be.
Once the pack is assembled, Peter will usurp Derek, preferably non-violently and in a fashion that solidifies Peter's credibility and showcases his newfound 'morality'. Given Derek's growing self-doubt, Peter might even convince him to abdicate voluntarily; it will only take a few cleverly-placed words over a period of a year or so, chipping away at Derek's already fragile self-esteem. Waiting another year will be nothing, after all the waiting Peter's already done. (Peter plays a long game; it's the only sort of game worth playing.)
When Derek is displaced, Derek's Betas will automatically become Peter's. Knowing himself as well as he does, and recognizing his own limitations, Peter had realized from the outset that he couldn't bring Scott and Stiles into the fold on his own. Peter lacks Derek's vulnerability and his emotionality, traits that cement pack-bonds and encourage interpersonal empathy. But after Derek has considerately done the work of recruiting and turning Peter's pack for him, all Peter has to do is to claim his pack.
The prospect of bringing Stiles and Derek together is also titillating, sparking within Peter an old hunger, the lust for other people's lust. Peter allows himself to think of it, sometimes - the two of them fucking like the youngsters they are, Derek hurting Stiles just right with the power of his thrusts, so that Stiles learns to enjoy it, crave it, beg for it, his eyes damp and his throat hoarse.
Stiles will love Derek's knot - Peter's sure of it - because Stiles very clearly needs to be filled beyond capacity, needs the hollowness carved into him by life's many losses to at last be replenished. The pain of the knot will make it even better, because the pain will make it real, make it tangible, the knowledge that Derek is in him and will stay there for ages, unable to leave Stiles like Stiles has been left before.
Derek, too, will thrill at keeping Stiles, biting the back of Stiles's neck while the knot swells and swells, forcing Stiles to stay motionless, every internal twitch of Stiles's body transmitting itself to Derek, making him shiver in turn. Oh, Derek will relish trapping Stiles beneath him, coming again and again inside Stiles without once withdrawing, until Stiles is sore and tearful and flushed and glowing, moist and trembling and feverish, smooth and hot all over, like a skein of fiery silk.
As a Beta, Peter can only plot and imagine. But as an Alpha, he can demand to watch. To see Derek curve atop Stiles as if to block him from sight, while still fucking Stiles savagely enough to be heard, the audible slap of flesh against flesh not nearly as obscene as Stiles's choked sobs, his liquid gurgles when Derek begins to knot.
Naturally, neither Derek nor Stiles would ever consent to performing for him, but perhaps if Peter were to use mind-force, perhaps if he were to make them forget...
He could have them fuck for his pleasure anytime he wanted.
Yet another incentive to be the Alpha, then.