What makes diverticulitis so bad that surgery is necessary?

Hole In The Colon: Diverticulitis occurs when a small pocket of the wall of the colon ruptures, thru which intestinal contents may leak into the abdominal cavity. The severity of the episode is related to how quickly and how well the body seals this hole shut. If it remains open, generalized peritonitis can develop necessitating emergency surgery to remove this ruptured area (with a temporary colostomy).

MAY cause bowel perf: Just like a 'ruptured appendix' is bad, so is a perforated bowel from diverticulitis. If the bowel contents (stool, bacteria) gets out of the intestine and into the 'body cavity' it can cause near-fatal peritonitis, sepsis (blood infection), shock , and eventual death. It is better to remove the appendix and/or the diverticular abcess before 'it's too late!'.