The show has made it pretty clear that when Jaime is away from Cersei he is a pretty decent dude, but once he sees her he can't help but to fall back into the toxicity. I think it's as simple as that, toxic relationships make people do things that dont make sense, it messes with your brain. He could have left intending to kill her but the moment he sees her he reverts back.

Cersei is similar. When Jaime is away she focuses on other things, but when he's back she is madly in love. That's why she was worried that he was bleeding. I saw something that couldn't understand why she would be concerned for him when she wanted Bronn to kill him.