I always kill him.
All this time he has tried kill me and my companions, spread all kinds of nonsense about me and overall made my task as difficult as possible. Then there is the poisoning thing, slavery, tyranny, and all the other things that have been mentioned several times. My warden hates all these things.
Even though he is a warden (human noble), he still has a strong desire for justice/revenge however you want to view it. He got satisfaction from killing Howe and views that Loghain made many of Howes actions possible.
Drinking some darkspawn blood didnt change his values and beliefs. He doesnt fully believe in some of the wardens ways of doing things and he never claimed to be a "perfect" warden himself. However he doesnt view being a warden as a punishment or total misery. He actually sees some honor in it and believes it is a chance to do great things, no matter who you are. So by making Loghain a warden, he would feel like he is rewarding him and the last thing he wants to do is reward him.
Loghain has tried to kill me all this time. Do I really want someone like that at my side? "We need him as much as we need a knife in our back" as Alistair says. I would propably kill Zevran for the same reason but in his case metagaming comes to picture or whatever you people call it. Loghain has made it painfully obvious that he is cabable of doing totally unexpected things for everyone. What would stop him doing something similar when I let my guard down?
Riordan has spent much time in his cell. Maybe too much. He propably has no idea what my warden has been forced to go through both mentally and physically so my warden doesnt listen his every advice. If he says more wardens would be a good thing then he can easily choose other cabable people.
My warden also fears that Loghain´s presence could have very poisonous effects to his companions and at worst his party would fall apart. He isnt the only one who wants loghain dead or at least hates him. He values his companions both as allies and as friends and maybe some of them even as loved ones. He will not let anything to ruin it all and he wont take any risks.
Before anyone says it, yes I have read the stolen throne and yes I have spared him one time but I didnt finish that playtime. They didnt change my opinion of him much. Sparing him left a bad taste in my mouth and I felt that I was a total ass to Alistair and a traitor.
In stolen throne I saw in him some of the things I hate in his DAO version like that arrogant thinking that he alone knows what must be done etc. I seriously think he should sometimes just shut up and listen other people´s opinions for once.
Other Batman thing that comes to my mind when I think of Loghain is "You either die a hero or you live long enough to see yourself to become the villain".
Some of you propably have valid reasons to spare him and thats fine. Perspective and free choice are great things. These are just my opinions. 