On my way to destroy all Demon Altars I found one to be inside my "quarantined " environment from corruption/hallow. I had decided to leave one of the altars on corruption alone for the purpose of having one still in my world, but this one I've just found within my closed environment is much closer to me than the one I originally set to be my last one.
My question is, is it dangerous to leave a demon altar inside my quarantine? Can it corrupt something by being inside? Sometimes I think they just stand there, but would it be wiser to leave a demon altar outside my barrier alive to be sure?