1. Some characters have more health than others. That is a fact of the game. I'm surprised you aren't complaining about the mana instead.
2. The blood pool, as a means of escape, is not that great unless near a turret. It lasts shortly, drains a good bit of health, and can easily be countered by stunning or slowing him before or after he gets the chance.
3. Yes, he can die. Just because you have problems killing him does not mean others can not. Massive health + life steal can apply to a number of other characters, especially with certain builds.
4. Annoying? That's a trait of being overpowered? Well, I guess Singed and Zilean are OPed now. Everyone can damage you, and guess what? Over so much time, that kills you, too.