Heres a link to show you how to get the achievement
You must either play as bill or have bill sacrifice himself for this achievement on the Sacrifice.
"Have Bill sacrifice himself for the team.
This Achievement shares its name with a movie.
This is the only Achievement which requires a specific Survivor.
This Achievement was probably made to encourage Players to copy Valve and sacrifice Bill. Alternatively, it could also have been made to help it fit with the Storyline in The Passing.
In Left 4 Dead the achievement is only obtained by the player who plays as Bill and successfully sacrifices themself. In Left 4 Dead 2the achievement will be obtained by everyone on the team if the sacrifice is made by Bill." http://left4dead.wikia.com/wiki/KILL_BILL#KILL_BILL