Simply having a thought about something evil or sinful isn’t in itself sin. We are sinful creatures, and even though God is in our lives, those thoughts are still going to enter our minds. The key is that you shouldn’t let the thoughts linger. If you continue to dwell on a sinful thought, then it becomes a sin. For example, if an image of a beautiful woman comes into your mind, that isn’t a sin. But if you allow that image to linger, and you begin desiring the woman, then it becomes lust, and that’s a sin.