Massacre Wurm might not be able to deal with big threats, but a few Massacre Wurm triggers could easily kill an Elves or Goblins player if they've managed to rebuild after the first time Wurm hit the field
This four-mana mono-green 4/3 makes every creature spell you cast with power 4 or greater cost two generic mana less, and whenever she attacks, each of those big creatures gets +1/+1 and trample until end of turn
I ran the question past my pal Jason, and he suggested Bennie might've been overshadowed by go-wide commanders with more colors, such as Jinnie Fay, Jetmir's SecondJinnie Fay, Jetmir's Second and Jetmir, Nexus of RevelsJetmir, Nexus of Revels
It's not infinite, since I need a card in deck for each point of damage I will deal, but it almost always at least kills one opponent