Confronting Downes is the worse mistake out of the two - in a moment of selfishness, callousness and greed he cut his life considerably short and unknowingly dealt poetic justice to himself. Of course, he became a better person due to the new perspective on life he gained from being terminally ill, but being a good person doesn't cure TB.

If he was not sick he would have beaten Micah, but that would have to make a new ending for the game, I presumably would enjoy if Red Dead Redemption included Arthur in the ranch, and stayed with Jack in the killing of Edgar Ross.

Confronting Downes. Even if the gang would've fallen apart, at least Arthur would've survived.

I mean he had no idea that his disease would spread until after. But saving micah he should have thought it through.

Confronting downes wasn't his fault, he was told to do it and didn't know it would happen. However, he didn't like Micah already and Micah had already made a lot of mistakes so should of left him to hang, so I think saving Micah was his biggest mistake.

Tbh when i first played RDR2, Micah seemed like a good guy. But, unfortunately he was evil. He was so convincing, like how he called Arthur "brother".

Arthur very well could have lived longer if he left the gang, even with TB. His time was sped up due to him not taking it easy. He was running and fighting almost every day and never settled down.