His line of work might require him to keep his identity hidden so that he could have a little peace when he was not in battle.
During his work for the Empire he may have become immune to violence and actually started enjoying it. He seems quite a shallow bad guy from what I’ve seen of him.
If you watch the scene just before you might him on disc 3 he says something about “my master is a fool”. Interesting, eh?
If you look at any close up of Zustava’s face, you can see burned skin underneath the mask. What probably happened was he was near death after being attacked by monsters, Craymen saved him and therefore Zustava owns him a blood debt, even though he doesn’t like the guy much.
If you are reading this, then
you are the better warrior. It is
my dying wish that you meet with
Lord Craymen at the Tower.
The two of you can save this world
Zastava
I guess deep down, Zastava saw, at his death, that Edge was the only one who could save the Panzer Dragoon world. Despite the fact that Zastava enjoyed killing people he must have truely seen some good in the world or else he would not have wanted to save it.