Evil food is a group of monsters added in SLASH'EM. Their appearance is the same as a regular egg, tripe ration, or food ration, making them somewhat similar to mimics. However the food will still move and are not concealed, making it simple to identify them either by their movement or using the far look command. See also evil coins.
Killing these monsters with a digestion attack will not give nutrition.
However, you have a 1/3 chance of getting a tripe ration or food ration from slaying the corresponding evil food.
Bad eggs are not related to rotten eggs, i. e. regular eggs old enough to give the "Ugh. Rotten egg." message and confuse you.
Killer tripe ration
Killer food ration