Dude... I know it seems harsh but please just put that thing out of it's misery. Put a bag around the cage and throw in a hose connected to your car exhaust, if you don't have the stomach to shoot it.

Don't stab it or beat it, and if you REALLY want to give it the most painless way to go buy a tank of nitrogen and put the cage in some sort of container to hold it in the N gas.

Iono, I had to put a porcupine out of its misery as a kid (it was half crushed by a truck, but still very much alive and screaming) and while it sucked I think back to 20 years ago glad I was able to put it out of it's misery.

Granted, the porkypine wasn't trashed and in a cage, it was in the street, so I had to use a square shovel like a guillotine... did the job.

Sometimes, putting an animal out of its misery is the most humane thing we can do when there is a 0% chance of recovery or rehabilitation.