If there are people we’ve harmed in the past, the Buddha says, develop thoughts of goodwill [mettā] for them. Keep them in mind, keep that attitude in mind, so that you won’t want to harm other people.

"As for the forms of doubt and uncertainty, one is the sense of insecurity that comes when you know you’ve done some wrong things and there’s the possibility after death that you’re going to be punished for them. It’s not necessarily the case that there’s somebody up there keeping records and they’re going to catch you and say, “Hey, look, you did this wrong, you did that wrong, and we’ve got these laws and we’re going to carry you off and punish you.” It’s simply a fact of action: You do something unskillful and it’s going to come back in an unpleasant way. For a lot of us, we don’t know this for sure, but there’s always the possibility. You can start thinking about things that you did in the past, where you harmed other people, and there’s always that uncertainty.

I’ve noticed that when people hear the teaching on karma for the first time, that’s the first thing they think about: “Oh my gosh, I’ve got all these horrible I did in the past. You mean I can’t get away with them?” Immediately dislike of the teaching. The way we can get around that, of course, is to make sure that we don’t harm others. If there are people we’ve harmed in the past, the Buddha says, develop thoughts of goodwill [mettā] for them. Keep them in mind, keep that attitude in mind, so that you won’t want to harm other people.

Because that’s the next step, of course: to develop thoughts of goodwill for everybody. You make up your mind that “Whatever harmful things I did in the past, I’m not going to do them again. I’m going to exercise restraint.” This is a different kind of restraint, not so much restraint in how you look at things, how you listen to things, but restraint in what you do, what you say, what you think, so that you can look back on your life and say, “There’s nobody I’ve really harmed. Or even when I did harm somebody, I’ve learned from it. I didn’t just keep doing it thoughtlessly.”

That thought helps to allay a lot of fears."

~ Thanissaro Bhikkhu "Prepare to Die"