I've completed the quest to kill the gang leader in the boil for Sebastian, but he won't join my group. He says something about me being too inexperienced. Does anyone know what the minimum level you need to be at to get him to join is? None of the guides I've read have said anything about level, only that you have to complete his quest.

Having done some searching myself it seems the only other requirement mentioned is your alignment, with him having a dislike of good characters. Is it possible your alignment is too 'good' and he just has an odd way of expressing it?

I am playing a rather good character (around 44 on the good scale) so maybe that's it. Maybe I'll try murderating a few bystanders and see if that helps.

Be sure to lure them into alleyways, disintegrate them discretely, or find a way to move the bodies and be nowhere's near the blood stain when there's a cop around.

He joined my do-gooder characters with no problem. I think you're just too low level for him, just like with Vollinger. Up a few, then check back.