You'd better not ask for trouble.

你最好不要自讨苦吃。

It's better not to ask him why.

最好还是别问他为什么。

You don't wanna do this.

你最好别这样做。

You don't wanna mess with me, trust me.

相信我，你最好不要惹我!

mess with 招惹，打扰