Unfortunately these things happen all too often, it's probably because the person/people have been in the guild so long they feel "too comfortable". Realistically you need to decide for yourself, afterall you know these people and can judge what is the best action.
It would probably be beneficial to whisper this person one day, out of a raid, and just talk to them, calmly and politely, just ask what the problem is, and see what response you get. If they're just being an ass then you really gotta get rid of them from the team, like many others have stated before me, it's not about 1 man, it's about the 9/24 others in the team/raid.
You really should nip it in the bud asap though because it will start to fester, and get worse over time.