It's his whole deal and I love it so muchhhh. He knows that he can't have Scott--not really, not in the way he wants, some dreams are impossible--so he needs to control Scott in every other way possible. He controls Scott's emotions, his relationships, his body through hugs and poison and mountain ash. He makes Scott wait hours for his death to make him appreciate just how much control Theo has over his fate, because that's how obsessed Theo is with this control. The obsession is bottomless because ultimately control can't satisfy his real need, which is genuine love from Scott--ironically the thing Scott has been offering him this whole time. Theo rejects it because it implies a vulnerability he refuses to submit to--what if Scott saw the real Theo, the ugly thing he actually is, and rejected him? Theo, like anyone with a transplanted organ, doesn't take rejection well.
So he tries to satiate that desire with power over Scott instead. To that obsessive end, Theo also needs to sabotage every relationship Scott has. Everything he does with a pack is a reflection of his unbearable desire for Scott. He claims that he wants the pack because they're all as dark as he is: "I came for the werecoyote, the one whose first instinct is to kill. I came for the banshee, the girl surrounded by death. I came for the dark kitsune, the beta with anger issues. I came for void Stiles! That's the pack I want. Unfortunately, it doesn't include Scott."
Unfortunately. Theo gives himself away. Scott isn't someone he can want.
Meanwhile, his treatment of the pack is about punishing Scott for being so desirable, about making the pack reflect Theo himself, about confirming his choices. Otherwise, he seems disinterested. He claims to want Lydia but dumps her body in the woods that same night. He seems indifferent to the fact that by the time he describes "the pack he wants," Kira is gone and Liam is slated to die that night. The important thing is that Kira and Liam are both killers--or they will be, it's only a matter of time--so they were never gonna be a good fit for Scott, and they'll hurt him, and that's what Scott gets for loving them, and he probably won't love them anyway, not after he sees them like this.
"Did you tell Scott you're planning to kill your mother?" he asks Malia, before assuring, "Don't worry, I'm not judging. I want to help you." He needs Malia to become more of a kin-killer, for Malia to lose Scott because of it. People who kill their families don't get Scott and Scott doesn't get them and that's the way it is. That's the way Theo wants it, actually. He's here to help.
Theo's most revealing treatment is of Stiles, the person whose relationship with Scott is most analogous to his own. They both knew Scott when they were young. Theo needs Stiles to accept that he doesn't need Scott--"you never needed him"--that they're not actually friends--"was he still really your best friend?" Stiles, like Theo, must decide to reject Scott: he doesn't need him, they're not friends, they were never friends, actually, Stiles is better off on his own. No one needs anyone. No one wants anyone. "We're all telling the truth now," Theo claims.