1. I'm not a fan of her voice acting. I'm not saying the voice actor she has isn't good, but I don't think she's the right fit for the character in mind.
2. The way she generally acts on missions can be very irritating, especially when she just straight up starts fights and loses her temper and ends up causing trouble in most situations.
3. The entire point of Arthur's character arc is transitioning from the cold hearted and mean spirited person that he was before to the more caring and humbled man he dies as. But one of his most consistent ideals has always been that revenge is a fools game. It makes sense for someone like John Marston to still not head his advice due to his personal anger towards Micah, as well as the fact that it is pointed out numerous times through out the game that John isn't a smart person, but I can't help but be annoyed at the fact that Sadie has the same desire for revenge, and yet if we turn down her begging for help to carry out said revenge, Arthur Morgan is a bad guy and she is devastated by this. Fuck off with that nonsense.
4. There are two key moments in the final mission that really set it home for me in regards to my feelings towards Sadie as a character. The first is when she comes to John and tells him she knows where Micah is. Before this mission, as I'm sure you remember, Sadie had a heart felt conversation with John about how she felt she might not feel comfortable about having him ride around with her anymore due to Abigail not wanting John to live that lifestyle anymore. And at the end, she decides that's probably for the best. But the very moment she finds out where Micah is? She completely disregards both that, and Abigails feelings on the matter, and tells him anyway, and just ignores the fact that Abigail is pleading with John not to do it to the point where she is almost in tears over it. Everything that happens in Read Dead Redemption, from John losing his family to eventually being killed, all happened due to Sadie feeling the need to carry out her desire for revenge and involving John in it, despite fully knowing why it's a bad thing for John and his family and that he's trying his best to get away from that life.
The second moment is when you find one of the guys Micah used to ride with and he tells you where Micah is. He reveals that he broke things off with Micah and wants nothing to do with him, that he wants to move past that and live a new life. If you choose not to kill him, John will take pity on him and say he can't do it, meaning he probably feels the man has truly changed and wants to live a new life. But Sadie, seemingly annoyed by this, immediately shoots him in the head and mocks John for being soft.
These two things were the tipping point for me. Sadie is not a good character. She's a massive hypocrite who seems to enjoy the bloodshed and violence, and while she is not the only one by any means, we're supposed to look at her differently as a person because her husband was murdered. Micah Bell's brother cut him out of his life because of his criminal ways. And yet despite this, we still look at him as the bad guy because of course we should, the guy is a monster and a complete asshole. I wouldn't mind Sadie being the way she is if the game wasn't afraid to acknowledge that very fact, and yet the only time she ever really makes any real light of it is after you invade the camp with her to carry out her revenge.
And hell, I don't even mention how we're supposed to just buy into her being an expert gunslinger because she had to learn to survive on her own with her husband and thus has experience with weapons (which is a very lazy way of saying that she knows how to use a gun). Most of these guys are as good as they are because they spent their entire life fighting as outlaws, to the point where some of them can hit objects such as bottles from a distance the moment they pull out their gun. But I'm expected to believe that Sadie is good enough to the point where her and Arthur are all that's needed to take on an entire prison guard force? Bullshit. The only reason I don't rag on her for that is because, regardless of how poorly explained it is, it would be hypocritical for me to be annoyed by that because I have no problem buying into the fact that eight of the Dutch gang members can walk up to a mansion filed with armed gang members, kill everyone there, set the place on fire, and all of them walk away unharmed.