Well... he can't. He can't heal the arl.
Obviously not, but anything within his capacity can serve as the next best thing, which is saving Connor.
That said, even if it was as simple as him performing some spell to undo the Arl's sickness, I would not have released him, because it would be like the cops letting a burglar go on his merry way because he was convinced to put the stuff back. I take the position that he acts as a man caught red-handed, not one acting out of the kindness of his heart. What matters is that he did it, and in the Warden's position, I must assume that if he wasn't caught, he would have likely run off and leave the Arl to his fate.