Yes, Quentin was very, very bad. But............
you have to remember that Quentin was beside himself, counting the days till the full moon and hating himself the next day. He was desperate for anyone to help him. Low and behold...it's Barnabas. He probably figured out that he would get Carl eventually, so why not let Barnabas do it for him. That way it's "out of his hands" legally that is, and besides, there was to much on his mind.
Besides, I have a brother, the only sibling I've got, and believe me, I'd shut that mausoleum door, and laugh if I knew my brother was in there. That would show him!
So, inconclusive, Quentin was very, very bad. He should have thought another way of keeping Carl quiet, (yeah, it's Collinsport, Maine for the love of God!).