If you have ill will for people, you’re going to act unskillfully around them, and that’s going to become your kamma. So to protect yourself from yourself, you need to develop goodwill to be universal.

"Goodwill [mettā] is a wish for happiness, a happiness that’s true, a happiness that’s blameless. And this wish is meant to be spread around. Again, it’s sometimes explained by saying that we’re all interconnected. We’re all part of one another, so we owe it to one another to have goodwill. But the Buddha never talks about who you “owe” goodwill to. He said it’s something you give to everybody regardless, because if you have ill will for people, you’re going to act unskillfully around them, and that’s going to become your kamma. So to protect yourself from yourself, you need to develop goodwill to be universal.

As the Buddha said, it’s a determination. It’s not something that comes innately to us to have goodwill for everybody. We’re very easily inspired to ill will by people’s actions when they harm us or harm somebody we love, or harm somebody we think is undeserving of harm. So you have to be determined to have goodwill even for people who’ve been evil, cruel, and thoughtless."

~ Thanissaro Bhikkhu "Always in Training"