Does cellulitis of the buttocks need to be removed and how to tell if the skin or muscle is dead and needs removal?

No: Cellulitis is an infection in the skin and is treated with antibiotics. Sometimes oral antibiotics are sufficient but intravenous antibiotics may be required for more severe infections. Only a doctor can determine if there is tissue that "needs to be removed".

Depends on exam: If early can be treated with antibiotics alone. However if there is an abscess one must have surgery to incise and drain the area.

Not normally: I suspect your question needs clarification. Cellulitis normally is a superficial infection and doesn't kill the skin or underlying tissues and as such removal or debridement would create far more damage than necessary. On the other hand if there is damage going on you may have something far more serious going on. See your doctor or a wound specialist.

No: It really is a clinical decision based on your physicians exam. If you are not sick with fever etc, and there is no significant hardness or bogginess to the area, antibiotics may suffice. If there is suspicion of underlying issue, or you do not improve, further studies may be needed to see what caused it, if anything must be drained etc.