You’re only still ALIVE because I made a promise. (Old video)

What the part about “you’re only still alive because I made a promise” means is that McKenzie was super mad at Jack for bulling her and taking credit so (she wasn’t going to murder him, nor beat up to the verge or death, more of a learn you lesson beat up) she was going to beat him up but Coraline had just gotten with him so she asked her not to beat her up. And since McKenzie and Coraline were really close friends she agreed not to hurt Jack ‘for now’