Out of all the other things that other people already mentioned, the thing that jumped out at me from all this (aside the fact that Eli is going to die) is the scariest thing about all of this: you can tell Jace is really getting off on being in complete control of someone and unquestionable to the point where he could decide their life or death based on his whim. It's a narcissist's wet dream, and especially scary since he's doing it to his own friend. Notice how he said "Stryker out" at one point instead of "Jace out".