There are more far fetched ideas such as T.rex hunting in packs. The evidence for this is T.rex's large brain. T.rex had the largest brain of any other dinosaur, and modern day pack hunters are mostly intelligent carnivores. Both predator and scavenger argument can be supported by the large supply of Triceratops fossils found that could have supported a bloodthirsty predator or have supplied enough carcasses for a scavenger.