Heart attacks aren't nearly this debilitating every time. My mother has had them and not gone to the hospital for days. Stupid of us but we live in the country so we don't always make the best decisions. If any of you ever have a heart attack or any heart problems, be sure you go right away to the hospital!
Heart attacks themselves range in severity and cause. Go hit up wikipedia for more direct information. CPR is designed to manually cause the heart to pump but Larry could have possibly recovered. If the heart attacks are caused by blockages, pumping may move the blockage and restore his movement for a time(possibly allowing the clot to move to the brain and kill him more instantly). If the heart attacks were caused by faults in the sinoatrial node, CPR is a little more effective and could allow him to live long enough to get some of St. Johns medical supplies. Ignore the fact that the walkers eventually overrun the place and try to imagine a different scenario.
All that being said, it probably is best to kill Larry but I considered it a choice in the matter of timing. Until we know for certain someone will turn, we should preserve their lives. I try to think, what if it were Clementine? I'd want Lily by my side to help me save her. Imagine if Clementine had been the one with muck on her back in the pharmacy in Episode 1. Kenny would have backed Larry in throwing her out. Katjaa is the one who likes Clem. Kenny just wants you around as muscle. Is that really who you want to side with?