Sin can be very deceptive...
Sin can be very deceptive. For many who think they "aren't that bad," this passage should really make them take notice. "Well, I've looked a few times at the lady next door, but it's not like I slept with her!" Well, even looking lustfully at someone is committing adultery in the heart. "Yeah, he made me mad, but I'm not going to kill the guy!" Well, even harboring anger at your brother or sister is a sin.
God isn't just interested in our words or what we do on Sunday morning. As this passage shows, he's interested in every part of our daily lives.