Why did dan kill Keith on One Tree Hill?

Answer 1

for a number of reasons. Dan thought Keith was responsible for the night he was trapped in a fire and almost died. Dan also resented Keith for being a woman that Dan still had feelings for. None of his reasons were legitimate. You can't justify taking someone's life, especially your own brother's.

dan killed keith because he was jealous that keith had karen

Answer 2

He was jealous that Keith was going to marry Karen and adopt Lucas.