Princess Celestia asked the element bearers to write her friendship reports. When she reads about how Twilight overcame Tirek, she is... somewhat less than pleased.
Twilight Time + Cutie Mark Crusaders = over the top antics. Add in a lesson learned and the Crusaders tried harder the next time. And the next time. And the next. Until, well, they didn't get their cutie marks...
Demons are not notoriously cheerful, happy, bubbly, or even remotely nice. Ponies are not notoriously cruel, mean, callous, or evil as a rule. So when Gakham, an imp from another realm is unexpectedly banished, what he finds is... hell.