I kinda think the idea of a conflict between Obi-Wan and Anakin is one of the biggest problems. Lucas seems to have tried to make their fractured relationship an indication of Anakin's fall while still suggesting they have this great connection, so you get this weird issue where they talk about being friends but always seem to dislike each other.
Why not make them actual friends? It's one of the easiest ways to make Anakin likeable. Anakin is just a normal Jedi who gets seduced by the dark side because he thinks he needs more power. I mean, they're supposedly in a huge war, so it would make sense for Anakin to yearn for greater control so he can stop all the suffering. Instead, he's corrupted and turns to the dark side.
Killing Mace is more important to me because of who Mace was and what he represented. In Star Wars, the Sandpeople were antagonistic and perhaps not even human, they were more obstacles and fodder really, but Mace was a character in all three prequel films. He had a name, he had a face, he was a respected leader on the Jedi High Council, so killing him, from a story standpoint carried more weight because the audience could better identify with him instead of a group of Sandpeople, who had just tortured Shmi Skywalker to death essentially.
But the whole idea was 'once you start down the dark path, forever will it dominate your destiny'. So, killing a whole tribe of people, even antagonistic ones, seems like it would pretty much set you up. Killing one person to save someone you love? I understand your points about Mace, but damn he's already done so much worse than that. Just feels ridiculous to say this is where Anakin turns.
Doesn't really seem like he 'turns' or 'gets seduced by the dark side' as much he just burns his bridges with the Jedi.
Lucas didn't do enough to build up to Anakin's massacre of the Sandpeople. I never got why he cut the Episode I scene of young Anakin getting into a fight, which showed his anger and propensity for violence
Honestly, I think this idea is a little too obvious. Just look at Luke Skywalker. He's a good person, but he's also impulsive, prone to impatience, and desperate to prove himself. In Empire these qualities push him towards a huge mistake because his friends are in danger. All that helps explain why he almost turns to the dark side in ROTJ, and it informs his powerful attack against Vader. You didn't need Luke to do anything bad to understand his anger, frustration, and willingness to exploit them.
Thinking you need your 'tragic hero' to commit massacres, kill unarmed prisoners, and generally act like an entitled dick to set up a fall from grace i
s too clumsy.