When you say heavier, I'm going to go ahead and assume you mean, "with higher kinetic energy". A heav car at 1mph can still kill but only if it's victim is unusually slow. A light car far at 100 mph will kill anything in its path.
Do you also support applying this of logic to other objects possessing fatal amounts of kinetic energy? How about bullets? We could just dispense with the whole notion of law and justice and police and just say, "if you don't want to get shot, don't walk in front of bullets"
People choosing to drive are choosing to escalate the energy available for property damage and harm to life. They should bear the responsibility of this, even if that means, *gasp* sowing down.