Also, a third moral would be not to abuse power.
Trixie wanted to amulet to get revenge and abuse power in the first place, but when it corrupted her, I assume she went further than she originally intended. When the amulet was removed, the corruption curse was broken and she realized the error of her ways. (Maybe something along the lines of "Wow. I took over Ponyville and banished Twilight when all I really wanted to do was show off and redeem my name as the Great and Powerful Trixie..." Her seeing the error of her harsh ways doesn't mean she's going to magically change her personality. She's an arrogant character.
Do you really need the show to handfeed you a "dear princess Celestia" at the end of every episode to be satisfied with the moral?
EDIT: Besides, morals are subjective anyway. I myself haven't agreed with every single moral the show has put out.