In "War of The Villains", Mother Gothel was hired by Mom's rival buisness, Planet Express, to discuss a negotiation. Irritated, Mom order her henchmen to activate her home defenses, causing Gothel to flee and leading Mom to assume she was dead.
When Mom left her company to inform Planet Express of the bad news, Gothel was found out to be alive, as she broke into Mom's Company and decided to settle things once and for all. Mom, shocked and annoyed, informed her that she "makes the calls to these dumb bastards". Gothel, having heard enough, sliced Mom through the stomache with a knife, killing Mom and causing Gothel to cease control of Planet Express.
It is unknown was became of Walt, Igner and Larry, but it is believed they hired Roberto to get answers on who killed their mother in Round 8.