Before you all crucify me (I know this topic has been heavily debated before) I was just wondering if Michael knew he was going to kill Fredo when he made up with him at Mama's funeral. My guess is yes, with the way he looks at Neri as he hugs Fredo, he did know he was going to have Fredo whacked, but he did that for show so no one would be suspicious later.

Or did he decide at a different time that Fredo had to go?

The question of when Michael decided to whack Fredo has been discussed here many times, but I don't think anyone doubts that the decision was cast in concrete by the time of Mama's wake. Attest Michael's look when he's "hugging" Fredo, and his whisper to Neri earlier, after the boathouse outburst--"I don't want anything to happen to him while my mother's alive."

It has been discussed many times, and there are a great many nuances. I think a part of Michael knew he was going to kill Fredo the minute it was confirmed that he was in league with Ola and Roth. There may have been another part of him that didn't really want to do it, but after the "second betrayal" which Michael exposed in the boathouse, Fredo was a dead man walking.

He didnt want to kill Fredo all the way up until the very end. He only cleared his head right before the actual gunshot. You could see him agonizing a few minutes before he goes to the window to view the shooting.