Alignment is removed in D&D 5E, so it's not a thing in BG3.
Paladins are pretty OP here. You do still, uniquely have to deal with Oath, as a paladin.
So if you take an oath to protect the weak, you can't go murderhobo or then you become Oathbreaker and get evil powers instead of usual. Oaths is like specs, you can choose one that will match your play, so it will generally be more relaxed than typical older D&D Lawful Good or suck thing.
Rest of the classes - do whatever, no alignment. Of course if you are murderhobo don't be surprised it will catch up to you in the end. But in a more realistic way, as in you don't do a favor for someone, don't be surprised you don't get that favor repaid in later act when you may need it to.progress more easily. Or if you kill or not rescue someone, obviously that path will be closed later.