First of all, I don't really see need in killing everyone just to get XP. On my third playthrough I've cleaned all passes from Fort Joy and I saw that killing mobs that are couple lvls below you are not that rewarding. At best you'll get 1-2 ok items, but you get like 200-400 XP per mob when you need 20k and estimating there is not that much of them that's not really worth it especially for the end game.
Also, want to mention that this game is not MMO or sth, it's cRPG with roleplay at it's best. You are free to do whatever you want.
And again, you can choose Lawful good and still walk in town and kill anyone you want because it's rewarding you with XP. In my opinion Tag system for chars is better then alignment since you as a player choose their way of being Barbarian or Noble or Noble Barbarian. You can be good or evil depending on the situation.
About companions. They have their own vision of situation. Not like good or evil, but depending on their origin story. If you want to roleplay you can talk to them to get their opinion on situation that happened and as I understood from Larian if you disagree on lots of thing they will be able to leave party or even attack you.
In D&D, as far as I remember (don't have lots of experience in it) you can have lawful good and chaotic evil chars in same party. It only describes their personality, maybe they will disagree on some stuff in how to workout some situations, but that doesn't mean that those chars will try to kill each other
