There's a lot left out in Solnit's article, see: http://www.modernluxury.com/san-francisco/story/alex-nieto-w...
How can you ignore the inciting incident in this case? Perhaps the victim was a man with some personal issues, but before being harassed by Evan Snow, Alex Nieto was a man minding his own business.
What made this situation special was that Nieto was armed with a taser. When Snow's dog assaulted him, as poorly managed dogs do, Nieto prepared to zap it with the taser.
Snow tried to tell Nieto not to tase his dog (the DA investigation says that Snow "begged" Nieto not to tase his dog, but let's allocate all charity to the dead and assume Snow was a dick about it). Nieto's problem stopped being the dog. Instead, he confronted Snow, threatening to tase him, and suggesting that the two fight. That, too, is detail we can only source from Snow's own report, but it squares with things like the text he sent to his friend and the reports he made to people outside the park.