In general, you are right in stating the facts, he is a thief and a vampire, he can be classified as a chaotic evil character. But in this case, both Shadowheart and Lae'zel can be classified as lawful evil characters. I appreciate the ambiguity of Lae'zel, Astarion and Shadowheart for their ability to change throughout the game and become, if not good, then at least neutral. Since I like to play for good people, I am proud of myself for the fact that I can change the world and reduce the amount of evil in it. Perhaps you take the fight against evil very literally? At the end of the game, Lae'zel has a silver sword and a dragon, and Shadowheart is able to save her parents. But Astarion has nothing left except hunger and the inability to go out into the sun.
