Consider strip 854 as a counterexample. After Malack gives him a piece of his mind, Tarquin does seem to realize he has crossed certain lines and apologises. Unless one dismisses that as a staged performance and faked sincerity, it gives a pretty good clue that Tarquin is perfectly capable of understanding when he does wrong - he just doesn't care, or doesn't want to backpedal, as long as it doesn't affect people he actually holds dear.
In my opinion, Tarquin knows what is wrong with his actions, but since he went on the track of a villain and dictator and so on, he just plays it by the script. He is on that way because he wants to live life as a king and go down like a big damn legend. If somebody makes him realise that there are better ways to be remembered, or that his life could give him so much more if he took a different approach to it etc., he may get swayed. Or may not, but it's certainly worth a shot.
Short story: I don't think the comic shows that Tarquin is irredeemable. So far.