Look, this is a matter of perceptions. Some people think a human being can consider whether or not another human's actions can be justified. Some don't even stop to think whther or not a Human can judge other humans, but simply judge right away. And some others try not to judge but to either forgive or endure by understanding the other Human.
In my case, I can't bring myself to judge other people when I myself have done enough mistakes to be judged too. It's clear that the massacre of a whole Clan is on a whole new level, though. I think the Uchiha fans and fanboys are too rash to criticize Tobirama or even Danzou, yet at the same time, it's not like I'm saying that Tobirama and Danzou had it right. Wherever you look at it from, both parties were simply trying to protect their families and their lives from what seemed to be a threat for them.
The Uchiha's Coup D'Etat was simply something that had to happen sooner or later, you put enough pressure in a wet sponge and it's only obvious that the water has to go somewhere. Tobirama certainly has a huge responsibility here. But if I look at him, I can understand that he lost countless friends and even brothers to the Uchiha, and he more than anybody else alive on his time knew how much of a threat another psychopath like Madara could be. His intentions were simply to protect the village that his older brother had struggled so much to settle, the people he loved and the foundations built by him and his allies.
Look at it this way, perhaps Hashirama had been able to let go of his negative feelings towards the Uchiha... Perhaps, Hashirama didn't even have such feelings in the first place, because young Madara proved to him that though he was an Uchiha, they had the same views. From Tobirama's perspective, all he could see is that the Uchiha killed his brethren and his friends, and that by chance his older brother happened to be merceful enough to let them be on Konoha. It's natural that he had a LOT of negative feelings towards them.
The complicated thing comes when your negative feelings spread to people of the same Uchiha blood, but that have nothing to do with the reasons why you hate the Uchiha at all. Like it's been said in One Piece, an unborn child carries no sin. The young Uchiha had probably no idea what their elders put Tobirama through, and I think they were innocent. Tobirama's unnecessary hatred made them turn into what he feared the most. It's clear that when you want to avoid certain destiny, being rash will only bring that destiny closer to you. That was Tobirama's mistake. Understandable, but a mistake nonetheless.
I wonder, though, if Tobirama passed away and gave the position to Hiruzen, why didn't Hiruzen change things? THAT's the real question. If he as a Hokage had have the balls to oppose the Elders and Danzou in a way that the Uchiha weren't discriminated so much, then the oppression wouldn't have caused a rift between that Clan and the village, and the negative treatment received from Tobirama could have been put in the past. However, Hiruzen simply let Danzou and the Elders take rash and absolutist decisions that they shouldn't have taken, which not only turned Itachi into a traitor but turned Sasuke into an avenger. Danzou, for whatever reasons, was making the same mistake Tobirama made. And the fact that Hiruzen wasn't firm enough to stop such things from happening, makes me think a bit less of him.
You can say that negotiations were no longer possible, but I call bullshit. As long as you really want peace, negotiations are ALWAYS possible. The complicated part is when you don't want peace, but superiority. Naruto could negotiate with Nagato even when more than half of the villagers were dead and his village destroyed. And we know how successful that negotiation was. So, ultimately, the Uchiha massacre was not only an unjustified method, but also incredibly stupid, unnecessary, and with incredible high odds of backfiring... Which it did.
Killing is never justified. It can be understood, it can be forgiven, it can be whatever you want, but not justfied. When humans are able to give life by themselves, THEN they can call themselves able to kill as well. But so long as life remains a mystery, death should be the same.
