Heh.. it depends on just how bad the season is.. Typically more people die in winter because that is when viruses and disease runs their courses. But then again, I voted on Summer, because I assumed through out the planet summer would be more deadly in some regions than in others. Like in Humid/sub tropical you have the four seasons, but in other places you only have two. Dry summers and Wet winters.. in climates like those where rain is scarce I would think more people would suffer and die in the summer months compared to their "time of plenty," when it rains in the winter and is cooler.
But yes, where I live Winter would kill more people than summer as it hardly ever reaches heat wave digits here that lasts... and ice storms, flu out breaks, power outages, are all more common in winter.