Anything could have. This wasn't about Reeva or her beauty and certainly not about how he felt about her.
This was entirely about Oscar.
I think this may be why it's so hard for some people to believe he could do this impulsively - pick up his gun and shoot Steenkamp knowingly through the door.
I firmly believe he wasn't really shooting her in his mind at the moment, just at her, in rage - which is how his psyche is able to keep up the charade of insisting it was all just one big accident due to his fear/panic/vulnerability/disability/stupidity (well, that and lots of rehearsal over the last year, I'm sure).
He's already proven himself to be a liar on the stand simply by refusing to take responsibility for the gun incidents. Lying is okay to him, even on the stand, as long as he keeps telling himself that it was an accident that wasn't his fault and that others made him do it...and then expressing this outward wailing of emotion (that quite frankly is creepy and odd and not at all the norm of someone grieving after a full year).
Reeva got in the way of Oscar at some moment during those hours leading up to her death. None of us will ever know why and maybe even someone on here has uncovered exactly the reason. But unless OP confesses every detail that night, we are not gonna know. And he's not gonna confess even though his version of events makes no sense.
Oh, and yes, all of the above is IMHO.