1. In 4.01 Mona exposed Wilden and Melissa as being the Queen of Hearts, as the two of them were dressed in identical costumes.
2. Wilden was being blackmailed by Mona meaning the footage she got of Melissa and Wilden could have been staged to make Melissa look guilty … either way, Mona was framing Wilden at this time meaning she could have easily blackmailed him into killing Garrett.
3. From our understanding the goal was to kill Spencer (death by tossing her off the train), kill Aria (drugging her then tossing her off the train with Garrett’s dead body), and of course kill Garrett, which was accomplished.
4. The Queen of Hearts is seen opening one of the car doors on the train and Jason was dressed as James Dean, not QOH.The person Aria stabbed with the screwdriver was clearly a guy, so Melissa is ruled out.
5. We believe the person was Wilden, he was in the QOH costume and although Jason was seen in 4.14 with a side wound that Mona tended to, it’s pretty much irrelevant. If Jason was in fact stabbed on the Halloween train, judging by the wound he had on his side, he wound have had significant blood loss which would have been visible.
6. The wound was a pretty deep wound and if we are correct he wouldn’t have acquired that wound from the angle of the crate, implying that he was pushing the crate from the edge as opposed to leaning his body over the crate and pushing with both hands placed on the sides. Also, he would have shown some sort of pain when they were being investigated. He showed neither and although the stab wound he had on his side looked like it could have been connected to the Aria stabbing there’s not enough evidence to blame him for trying  to toss her off the train.
7. Also, Wilden’s murder could have been revenge for putting Aria in the box, meaning that if Ezra found out that Wilden was involved in that scenario it made killing him all the more better.

We just don’t believe he was the guy that Aria stabbed, we could be wrong but for us there is not enough evidence that would prove it was him other than that wound he had on his side. Anyway, we believe it was Wilden because Mona was blackmailing him and he was dressed as QOH.