Well, I think it's one of the minor issues of type-casting all the Toa, like how water Toa and Matoran were always female.
So, accepting the humanity of the Toa, like is it enough, or not how you expect?
I normally play almost entirely human, but it's fine XD the GMs on the other one never set a strict definition so eh. :P
And the way I always took it... It's a byproduct of the flame energy. Like... Ta-Matoran are very resistant to heat, that's part of their "biology". The anger is caused by the Toa energy much like our emotions can be influenced by hormones or chemical imbalances. That's not to say that those imbalances are part of who we are, but it's a part of our mind being bound in the body it is.
Don't make me get into a philosophical soul argument, I beg XD