Your wife is being violent with your child and you're wondering if you should leave? Just do it. If you're afraid of her, how can love dwell in your heart? As someone else mentioned, it is not her job to discipline your child.
And perhaps not focus on another relationship until your kid is eighteen. Disagreements concerning step-kids causes too many problems and is actually one of the reasons that second marriages fail at an even higher rate than first marriages.
I hope the next time you post, you've gotten your kid away from your wife.
I know the first few weeks of separation are brutal...but you will get through them, I swear to you.