People criticize the writers for making Cato "evil" for the sake of evil, but I really don't get it.
He has great issues with the Khemians because he got brain damage fighting against them. Furthermore, that brain damage might have grown into some sort of psychosis or mental condition that made him hate them even more, in a sick, demented way - even though he was able to hide it for so long. That's also his motivation to kill Amicus' father - who was trying to make peace with them. It's quite simple.
As for being homophobic, well, it's not like he comments it more than once. It seemed to me that it was more of a pretext to accentuate his actions further, not even disgust, though even that would be perfectly coherent with the society he was born in.