Harry was a bit of dick to blame Snape to be honest.

Yes, Snape did taunt Sirius about not being able to leave the house, but how Harry can assume that that was what led to Sirius' death, I don't know.

Both Harry and Dumbledore were at fault for Sirius' death - If Harry hadn't pissed off Snape by looking at his memories in the Pensieve, and had actually bothered to learn Occlumency, he'd never have had the vision of Sirius being tortured and therefore would never have gone to the Ministry.

And obviously Dumbledore should have been open with Harry. He should have told him that Voldemort would try to lure him to the Ministry. But fair play to Dumbledore; he admitted his mistakes at the end of OoTP.

I think we should also consider the part that Umbridge played in Sirius' death as well. I'm kinda surprised that Harry didn't think to blame her as well, given how much he hated her. The way I see it, Umbridge made Sirius' death more tragic; she cut off Harry's communication to him, and the next time that Harry sees Sirius, he dies. Just think, if she hadn't forced Dumbledore out of Hogwarts, things would probably have been different. If she hadn't been watching over the fires or reading Harry's mail, he would have been able to communicate a lot better with Sirius.

But that brings me on to my next point - the mirror that Sirius gave Harry. Harry just neglected it completely. In his panic after his vision of Sirius being tortured, he didn't think to use the mirror to contact Sirius, which is another reason that Harry was responsible.

So I'd say Harry was largely at fault and so was Dumbledore, and I think even Umbridge was more responsible for Sirius' death than Snape was. I just think that Harry was quick to blame Snape because of his father's and Sirius' hatred of him.