In Maniac Mansion's sequel, Day of the Tentacle, Weird Ed has a blacked out repressed memory where all he remembers is that it somehow involved his hamster and that it was a problematic moment for him. He also has taken up stamp collecting to apparently deal with his anger issues over said incident.

This heavily implies that the hamster event in Maniac Mansion canonically happened, despite it being an optional Easter Egg in the game, where Syd or Razor can microwave his beloved hamster, causing it to explode. The fact that he feels the need to deal with apparent anger issues from that event also implies that the further optional event where someone showed him it's remains canonically happened too, which causes him to kill that character in a fit of rage.

Maniac Mansion had you choose three out of seven playable characters: Dave, Bernard, Jeff, Michael, Razor, Syd, and Wendy

So, who was killed?

In Day of the Tentacle one of the playable characters is Bernard, so it couldn't have been him, though he was apparently one of the three that canonically had entered the mansion in the previous game, and if this event is canon then one of the other characters had to have been either Syd or Razor, or both. While only Syd or Razor would be willing to microwave the hamster any character can carry it's remains, and if Weird Ed runs into that character while they are doing so he kills them on the spot.