For a while, this achievement was frustrating because I kept playing the campaign over and over and yet it never popped. The reason for this is because the Left 4 Dead servers are glitched, and while these achievements may be incredibly simple to unlock in Left 4 Dead 2, that's just not the case in Left 4 Dead.
So what you need to do is get two controllers and start up a System Link Match. In order to do this, you unplug your wireless adapter/modem/whatever is giving you internet and sign in your second controller. (You can't use a guest, you must use a live account) From there, go to Campaign > Play With Friends > Create Your Own Lobby.
So you've made your lobby. Set it to The Sacrifice, and pick the last level. Now pick your character. By playing as Bill and sacrificing yourself, you can also unlock Kill Bill, however this also works if you get a buddy to man the second controller and have him sacrifice himself (Which is what I did.) However if you have no one who can play with you, this is what you do...
Play through as your main account, and set the second to idle. (Start Menu > Take a Break) Set off the generators one by one. (Or if you have a buddy, set off two and then go for the last one) I have found that rushing often will get you killed, even on easy. Tanks are serious shit, and this level can often spawn two at the same time, so be careful.
After you have set off all the generators and defeated the tanks, the bridge will lower. Run across and hit the switch. One of the generators will fail. Now either you or your second controller can run out and restart this generator.
The reason you sign in two controllers is because at least 2 human players must be alive - one of them on the bridge, and one of them to go sacrifice himself. The game states this is you attempt to do this on your own.
And again, if you have Bill sacrifice himself for the team, you will unlock both achievements.