Actually, yes they probably would have found some other pretext for attacking women. There's a certain type of angry young man whose life revolves round finding a "cause" to justify a good punch up. It could be football in the 1970s, or the miner's strike in the 80s, or extreme right wing (EDL type politics), or militant Islam. And these guys don't keep their violence in a neat box - they also glass random blokes down the pub and commit domestic violence in the home. And as various people have noted, the blokes who attacked Maria weren't trans in the sense of being transexual, they were the "dye my hair purple and loudly proclaim I'm queer, but make no other changes to my life whatsoever" variety of "trans."
Also, what "rights" would self-identification give genuinely trans people that they currently can't get through following the current GRA procedures? I'm genuinely puzzled by this. I want people who are genuinely trans to be able to have access to jobs (barring the very few with exemptions from the sex discrimination act on the grounds of necessary sex segregation), I want them to have access to housing, I want them to be able to go about their lives free from harrassment and the threat of violence. And guess what - they currently have those rights.
What I don't want is an "imaginary right" to declare (while wearing a full beard, being physically larger, stronger and faster than women, being sexually attracted towards and violent towards women and retaining your penis, etc) "I am a woman now", and that declaration having no checks and balances whatsoever. Utter the magic words, and you get to compete in women's sports, gain access to women's shelters, get reassigned to a female prison even if you've raped women and girls.