Moral Dilemma: My Judgement

So, some very good comments. Glad to see you thinking deeply about it. It's clear from the start of this quest that our three suspects all carry some portion of blame. All three are at fault and yet one must be found guilty. I'll admit, I did spend a lot of time in thought before making my decision.
It would be very easy to be lenient on Quint, seeing how his fault in this matter was largely passive just as it would very easy to be harsh on Karlat, seeing as how he 'planned' this mass murder. And then Belial as a wildcard. If all three men carry some blame then I realised I would have to look deeper.
Firstly, responsibility.
Is Quint responsible for this whole mess for turning a blind eye to his brother's attempts to summon Belial and seek out undeath? We are led to assume Karlat tried a few times unsuccessfully to summon Belial before managing it and thus Quint had plenty of time to wise up.
Likewise, if Karlat hadn't summoned Belial then this whole mess could have been avoided...
But if Belial hadn't lied?

Well, my next stop was to consider motives. In this sorry saga it becomes clear that Karlat acted out an instinct to protect the people of Charwood. Maybe choosing to do so 'at any cost' was the wrong choice, but ultimately he intended no harm.
Quint acted out of the instinct to bond with his brother and the children of Charwood. Maybe he hoped Karlat had changed his ways and was hoping to spread goodwill throughout the town?
Belial on the other hand acted purely out of malicious intent. He was instrumental in causing these events to transpire and indeed events played out exactly as he planned. There can be no doubt as to his intentions.

Which brings us back to a few comments made by you fine chaps.
Ches asked whether the stone is to blame for being heavy and the answer is of course no. But is Belial culpable for being evil? In my eyes, yes. either Ches or K also mentioned fallen angels and those who have played the game will remember that awful feeling of watching the paladin Lady Aribeth fall from grace and walk the path of evil. Sure it might be in Belial's nature to be evil, and such behaviour is to be expected of him but he still has free will. Just as it was in Aribeth's nature to fight for the greater good, so too was she capable of unspeakable evil and just because Belial is inherently evil does not mean that he could not have acted good, or even neutral in this situation. No one forced his hand, he was not misled as Karlat and Quint were.

After taking all this into account I placed the blame firmly at the feet of Belial. Although there exists a small cabal of gamers who blame Quint for allowing Karlat to summon Belial and an even larger slice of the nwn community who blame Karlat for summoning Belial in the first place I personally think Belial is the one most to blame. It was he who directly planned this crime, it was he who pitted brother against brother and he who forced one of Lathander's own servants against him. Karlat and Quint were unwitting pawns in his game. sure they should both have known better and are not blameless by a long shot and maybe the road to hell is paved with good intentions but ultimately I find them both guilty of stupidity and ignorance where Belial is guilty of masterminding this whole affair. He is naught but an opportunist, taking advantage of the naivety of lesser beings to bend them to his own will and thus I condemn him.

Do bear in mind though that this is merely my interpretation and does not lesson the votes against Karlat thus far expressed by my fellow judges. ;)

Thanks for the comments. That sure was fun.