My Warden let him live. Honestly, I don't see any reason to kill him beyond, "OMG DARKSPOON MUST KILL!1!11!!!!"".
Causing the Fifth blight wasn't his intention. Now, I haven't read The Calling, so maybe I'm missing out in some important information. I think that his whole intelligent Darkspawn thing is amazing and potentially a more permanent solution to the Blights. I prefer to work with him rather than kill him.
Edit: Gosh now I REALLY want to find out what happens in the Calling that makes you all want to kill him... he seemed reasonable enough
.
2nd Edit: Okay, just read the Calling's wiki page. The Architect's plans were generally unhealthy (understatement of the year), but I don't see him as being evil or worthy of my Warden's blade. Anyway, if the Wardens keep a close eye on him, things should be fine. It's supposed to be an uneasy alliance, right? You can let him live without fully trusting him. I think the Wardens can probably benefit from his research and knowledge.