You might as well be nice to everyone, because in the end if something happens to someone your mean/rude to, you are going to feel like a complete asshole in the end.

Knowledge Minus Ego Equals Wisdom

How people treat you is their karma; how you react is yours.

Never get close to your enemies, you might start liking them.

Take no duty of the Guard lightly. Friends must not be enemies
Just as enemies must not be friends.
Discerning the two is a life's work.

The past is called the past for a reason. If you are constantly looking behind you, your eyes aren't on the road ahead. You don't drive the car that way, so why should you live your life that way?