The difference is we have unquestionable proof that he was motivated by Brown and Garner given his tweets. Thus far we have no proof that he was motivated or inspired by protesters chanting for cops to be killed or any other kind of anti-police rhetoric.
Unlike you I actually attempt to remain consistent in situations, I don't abandon principle whenever it suits my political needs. My stance on these kind of things have been consistent...whether it's been foolish liberals trying to blame tea partiers for the death of a census worker or declaring that islam caused someone to shoot up the holocaust museum or people jumping on the NY posts suggestion of who the boston bombers were or, in this case, people trying to claim for sure what motivated this guys attack.
We don't know the facts yet, and I'm not going to say something is definitely the case when there's no facts what so ever to back it up as definitive.
Thus, there's still a legitimate question if he was motivated to any degree by "anti-police rhetoric". I would think it's more likely to be "yes" than "no", but it's absolutely not a definitive thing at this point.