I notice that at least one article which went into detail about the birds injuries said that they were multiple puncture wounds and lacerations to the bird's abdomen. It sounds to me like someone went at it with a knife. And they wouldn't necessarily have needed two people... one guy with a heavy leather glove suitable for handling raptors could have held it by the legs while stabbing it with the knife in his other hand. It would have struggled a lot, but a few good stabs and it probably wouldn't have had a ton of fight left in it. At which point the killer probably would have dropped it and left. I would surmise that any bite wounds from a feral cat may have come afterwards, while the bird was already lying on the ground and dying.

If the bird was a permanent cripple with a broken wing, then it being caught on the ground by a predator is even more suspicious in my opinion... usually raptors with broken wings like that are kept in pens where they have ramps and platforms they can walk up to hop up into a branch or constructed perch of some sort. Their talons are not really built for extended time walking on the ground, so without such a setup it could have developed problems with the pads of its feet. That's avoided by giving it access to a branch-like object it can wrap its talons around and perch on.