I haven’t posted in a while, so I thought I’d drop something in for the hell of it. This is the Epicurean Paradox, which is a form of modus tollens logic. The fact that you can simply state the question shows that the premise is false.
If God is willing to prevent evil but not able to, he’s not omnipotent. If he’s able but not willing, then he’s malevolent. If he is both able and willing, whence cometh evil? If he is neither able nor willing, then why call him God?