Hi! I'm going to look into that. Are you able to buy the knife? Does it appear as an item you have during the game (the icon is colored and not a black silhouette)? Is the item being used or just nothing happens when your press e with the knife selected?
After the knife is purchased and then selected as the item in the bottom left during the game, when pressing "e" an animation should play. The knife shouldn't be used when any of the other ending is being played (the losing due to anger being too high, etc.). In my game the game has only one ending with the knife, not like in the Mr. TomatoS.