What would you do if give the option to travel back in time to murder the first person with a genetic disorder or communicable disease and guaranteed that that disease will never make an appearance? AIDS for example. If you could go back and eliminate the very first person with a solid promise that it would never show it's face, would you? What about down syndrome? Would you wipe that from history with murder? Or hemophilia even? The common cold? I would, with the guarantee of course. Animals and insects know what's up.. They'll give a sick comrade the boot to save the rest of them. Some of them know and go far away to die, to protect the others in a way. What if a new disease popped up today, one that could threaten millions, and you had the option to execute the very first person and stop it forever, would you?
Butterfly Effect. edit: and no, we have a serious over population problem as it is, the last thing we need is for every one of these resource sucking homosapiens to thrive and stay alive.