From recent news items, it seems the person had actually been attending the gay club for some time, and was probably gay himself. Also, others have said he desperately wanted to be accepted and to make gay friends, but his strange and disturbing personality put people off. I'm now tending to think his terrible actions were a result, not of homophobia, but of social rejection and revenge.
It appears obvious that he was very mentally disturbed and felt isolated, so retaliated with the appalling results of a mass killing.
I actually feel some pity for him. The Dunblane killings were carried out due to a similar 'rejection' anger.

As a teacher, I've come across one or two pupils who desperately sought 'acceptance' by their peers but were too strange/weird to make any friends. They invariably turned nasty and damaged other pupils' work, started aggressive fights and lashed out. One or two were expelled from school. I suppose if they had had access to firearms they may in later life have exploded in wrath in the same way. In other words, 'If you can't join 'em, beat 'em.'

I'm just trying to understand how this awful situation comes about. If only these individuals had received help earlier, (and love,it must be said) they might not have erupted in this lethal fashion.
A terrible tragedy for all concerned including the perpetrator.