It wasn't Rin's death alone that drove him to the dark side. He was also heavily influenced by Madara who promised him things. Do you think Obito would have been evil if Madara never got to him? He'd be dead really. Not only that, but we have other villains who were evil for even lesser reasons.
He made not have been, but its academic at this point because he did and Obito made the decision to follow him and continue to do so even after he died.
I suspect when Madara's role came out, especially when it was revealed he was behind Rin, that this was the kind of blame shifting Kishimoto had in mind. I think their both culpable. Of course, Madara himself was manipulated. And Kaguya appears to be as well.
Hence the ongoing complaint that no is ever held responsible for the choices they make.
As well, there are so many characters who have tragics pasts and didn't go evil. Let's make a comparison.
Why did Madara become evil? He became evil because his brothers were killed in a war and he was jealous of everyone loving Hashirama.
Hashirama lost brothers too in the same war and he still became good. So basically, jealousy is what drove Madara evil.
Sasuke lost his family and became an anti-hero seeking revenge. Then he turned evil because Danzo forced Itachi into that situation. Then he became an an anti-hero again for no real reason.
Naruto has no parents, everyone hated him in the beginning and saw him as a monster and he was mostly alone. Did he become evil? No.
Gaara too. Whole clan hated him, saw him as a monster, etc and he eventually became good. (Not only that, but both Naruto and Gaara both had parents that wanted the best for them and nobody listened.)
Obito became evil due to Rin dying and Madara's corruption.
Tsunade lost her lover too and she didn't become evil. Just depressed.
Part 2's problem in a nutshell. No responsibility to be found. It is always someone else's fault.
So basically whatever villain you want to bring into the equation to explain their reasons what you deem "acceptable" is really a subjective view and I could easily say all reasons for their bad behavior is unacceptable with them all being spoiled brats and throwing a hissy fit because they didn't get what they wanted. Orochimaru's reason for being evil is even less than Obito's reasons, but I don't see anyone really complaining about this one....and he was a major villain in the beginning. Kabuto also had weak reasons for being evil and he was only semi-tragic.
So here is a better idea: what has been the best reason why some turned evil and we shall go from there. We will go down the list from best to worst. I'll say right now, Obito's reason are not the best, but they are not the worst either.
Is there any good reason to become evil for that matter? As I said above, we all have breaking points and some are more harsh than others. Obito's case is not too far off from what I have seen of others. I know this guy who lost his father to cancer. After that, he became a total ass and sold drugs on the side even getting into gang warefare. I have seen people become bad over less reasons. Trust me.
In criminal law, it's what is known as a mitigating circumstance. Why someone might get life over death, 20 or 30 years instead of 60, or a reduced charge. The thing to note though, is that they are still punished because it doesn't make their choice right or justify harming someone else. There are also some cases where the wrong is so egregious that nothing can really mitigate. Obito is well into that territory. His basis isn't the flimsiest, Danzo's probably is, but his crimes are so awful that I cannot bring myself to care. He wanted to take EVERYONE's life from them no matter what they felt because of his own opinions on the matter.
If Hitler had lived long enough for the Nuremberg trials, would anyone have really cared why he murdered 6 millions jews? I suppose on a "let's find a way to make sure it never happens again level" but in terms of deciding his fate? I'd bet the only concern was that he, in fact, had that many people put to death.
After thinking about it... this chapter basically explained itself. Hand Waving Obito's crimes is no different then what's likely to happen to Sasuke
You go it. I think Sasuke fighting on the good side (without really being good yet) is much like Obito taking it for the team. It'll be the reason Naruto can hand wave Sasuke's culpability.