I don't deny that he may have been a spoiled, rich, brat and can be extremely selfish, but there are moments where he doesn't seem like a chaotic evil or just plain evil character, he remains neutral. There is no affect on him. Based on my character's interaction with him, he just seems extremely selfish and all he cares about is his freedom. If he needs to help someone to further his freedom, he'll do it. The feeling I get from him is, the more loyal you are with him, the more loyal he is with you. Especially if you side with him a lot, he'll change the way he talks to you. Of course this goes with any other companion.
I like it when characters don't become...caricatures. So having companions with multiple alignments, inherent and inherited makes them more interesting to me. I've thought this a little when people criticize Shadowheart for being 'inconsistent'; "wait so you want them to be more moustache twirling?" . I will say Astarion is the most murder-hobo character we get, but I'm still interested in how he got to that point, which is all I'm really asking for in an evil companion.