This argument is absurd. If you believe that CDGing an evil, sleeping opponent is evil, then all these other things must also be evil:
1 - Attacking with anything other than subdual attacks (What if you crit and kill the demon? Oh, can't kill anything, that'd be evil).
2 - Letting a dying evil enemy die while you loot him (or do anything else). Oh, now you not only killed him, you killed him while he was helpless. Definately evil, right there. That's like infecting-the-local-metropolis-with-vampirism class evil right there.
I could go on, but I won't.
Two misconceptions here: a) the enemy was helpless, b) killing is evil.
a) The enemy was not helpless. Given that he still had comrades in arms still fighting who could've easily woken him back up (doubly so because you were outnumbered 2:1), the enemy was still a very real and tangible threat to the party. Secondly, it's not like you could spare the resources to babysit him while you were still fighting bad guys. What if the battle lasted more than a minute, and he woke up and either attacked you again, or ran away only to rob/kill innocents because of your mercy? If you have an opportunity to stop an evil person from committing evil deeds, but conciously don't prevent him from doing so, how is that anything but an evil act? Even if you snap his bowstring or something, if he wakes up and runs away, he can easily get another weapon (club or sling, anyone?) and continue on his evil ways.
By the way, even if this were the last person to fall, I wouldn't call it evil to CDG him. The guy's evil, he lost, too bad. It's not a particularly lawful thing to do (depending on if you actually have an honest source of official justice nearby), but it's certainly not evil.
b) In my (3.0) PH, the first thing is says is that good characters preserve innocent life while evil people kill innocent people for fun or profit. It doesn't say that killing in itself is an evil act (which would be absurd, is preserving a demon's life a good act? I think not). Killing an evil person who is still a threat to you/your friends/other innocents is a neutral act at worst, or a good act most of the time.
I don't understand why the helpless part matters so much to so many people. If anything, it would matter more for Law/Chaos than Good/Evil. The idea being that taking the role of judge/jury/executioner is, in effect, disrespectful towards legitimate (assuming it exists) law that could honestly try the evil person for his crimes. If there doesn't happen to be a legitimate (and honest) authority in that part of town, then I would hope that the party would take it upon themselves to be the judge and jury. If you witness an evil person doing evil things and there is no one else around to do anything about it, good characters SHOULD perform a reasonable punishment or take some other step to prevent him from continuing to be evil.
In other words, if this was a lawless land, then I would call the act neutral good. If this was in a civilized land with a reasonable expectation for justice, then it would be chaotic neutral at worst, and certainly not something that would instantly change your alignment.