I love Trixy's design and her character, she wasn't all that likeable but her appearance in this season has made her more popular. The fact she made the effort to apologise rather than maintain her pride was evidence that she might be changing, but rather than leaving the past in the past she let it get to her and sought a way to humiliate Twilight. Revenge is NOT an answer.
Even then, she did not solely punish twilight alone, but Snips, Snails and other ponies who were simply on the sidelines of the previous confrontation. Although it was brave of her to aid Twi and apologise to her face - it wouldn't be enough. Trixy abused other ponies with the powers she gained to abuse Twilight. She should be forgiven, but face the consequences of her actions behind bars first.