Well, let me start by confessing that I do presume that civilians don't murder complete strangers on a whim. Nor do I assume people are white supremacists just because it allows me to hold onto a narrative that there's no other evidence for. So if somebody dies in an altercation, unless there's evidence to the contrary, I start with the assumption that it was an accident.
But there's also the fact, as shown at the link that accompanies that sentence, that Penny was seen putting Neely into the recovery position after he'd let him go. And that not a single person on that train tried to intervene with the restraint. In fact, as I mention, two other passengers helped him. Or are we presuming they're all white supremacists too?
Vazquez (the journalist on the train) says that he thought Neely had just passed out, not that he'd died. It's very easy for you to say there was no justification for his actions. But it appears that everybody who was actually there, disagrees with you.
Chokeholds are dangerous things. And in the heat of the moment, it's frighteningly easy to go too far. That's why the police aren't allowed to use them, for example. There's a very fair argument to be made that Penny shouldn't have used one either. That's why he's been charged with manslaughter. But that's a far cry from saying he deliberately murdered Neely.