Stewie Kills Lois is episode 4 of family guy season 06. It was originally aired on November 4, 2007.
In the 100th episode, Lois and Peter go on a cruise, leaving Stewie and the family behind. Upset with Lois for leaving him at home, Stewie vows to carry out a diabolical plan. When Brian challenges him to actually do it, Stewie stows away on the ship, finds Lois and…well…the title of the episode says it all.