Originally Posted by Viva
A living creature's purpose in life is to create more life, and fighting to the death with its own species seems kind of backwards from a psychological standpoint.
It's not backwards....the dominant male will pass on the stronger genes to the female. And this propagates the species better than if the weaker male were to mate.
And as far as "fighting to the death" as you mentioned, this is mostly in captive since in the wild there is much more swimming space and the weaker male will more often times escape or swim away before allowing the other dominant male to finish him off.