Meh. The chimera is understandable, but not reasonable.
Even if I agree with them and say that they're still human, for example, If I were a knight of The Empire, I would still kill them for being selfish murderers, vandals, and thieves. Perhaps they were once innocent, good men, and perhaps they indeed hold no fault for their being turned into monsters, but they are not good people anymore.
Besides, how could I trust what they say about the Dragon's Treasure? For all I know, the evil mages who created these chimeras implanted the idea within them, and an even more terrible result would come from allowing them to take and use the treasure. This man is just the leader of a band of mindless murderers and barbarians recklessly attempting to use a great magic treasure they don't fully understand for their own gain.
The hope the chimeras have sounds nice, but it's still a cleaner end to simply kill all the chimeras.
It's understandable that they didn't kill themselves; they aren't a warrior who believes in a code of honor, but they are not reasonable for immediately choosing brute force, just because the defenders of a fortress refused to hand over the single treasure they are to protect.
They are willing to kill others to help themselves, so the protectors of The Empire will obviously kill them, if they catch them.