This is a horrifying story and I can't bear to watch the video. My heart goes out to the poor victims of this attack and their families. I hope they're being supported. 
I think a psychologist would say that at the root of the rage and anger response is fear.
My ex was bullied as a boy and is convinced it was because the other kids somehow 'knew' even before they did (I'm not sure about that bit - from being bullied myself, I just think some kids can be cruel sometimes due to a range of issues they're trying to deal with).
At its root perhaps it is a fear that they don't 'pass' despite what their oh-so-kind friends say on Facebook, and they will be found out...when your innermost identity is at odds with biological reality, it must be difficult.
Ex is now in the US and terrified that 'people might kill them' because of being a transsexual when they go in the women's toilets (where they of course deserve to be, with their newly minted feminine pronouns
). Maybe with the US gun culture that's not so daft a fear, I don't know. But fear certainly doesn't excuse violence in response.