I'm not even really buying that he hated white people in general.
From heavy.com:
On Saturday, Johnson posted a video of people slaughtering dolphins to a Black Panther Facebook page, writing, “Look at the joy on their faces. Why do so many whites (not all) enjoy killing and participating in the death of innocent beings?,” according to CBS News.
I mean, he bothered to type out "so many whites (not all)". People that don't even hate white people rarely bother to explicitly state "not all white people" (not that they should feel obligated to or anything). Yet, he explicitly stated "not all white people."
Furthermore, if he was out to kill white people in general, why did he only shoot police? I imagine the area was packed. Probably some white people walking around that he could have shot in-between shooting police. Yet, he didn't. He only shot police.
Also, people seem to be taking the police chief's word for it. I wouldn't be surprised at all if he made up the part about the shooter saying he wanted to kill white people in general. Easier to get people on your side that way. Makes white people afraid.
Now, I think if he did, in fact, hate white people, it would be understandable given the reasons you've just pointed out. And if he was, in fact, targeting white people as a whole (though I highly doubt it) then it's something I would be opposed to. But again, I don't think either of these things are true.