They didn't leave him alive out of mercy, either. They plugged up his airway and pinned his arms down, and because of the collar no one else could help him either. They left him alive so Allen could watch him die.
So... you have to understand when I tell you that we can't leave this alone. The General is the strongest of us. They could do the same thing or worse to anyone in this dome.
[You get what he's after, right? There's no guarantee the Exorcists could kill Road and Tyki, but they can't let them run around to do this again.]