Alright, so I'm asking this here because I figure that you Tevinter-loving players also have an interest in Alexius and will be able to answer this. I've been trying to think of the various bad things that Alexius actually does which might be enough to warrant his execution. For me there is always this element of pity for Alexius because of his son that I find it difficult to execute him, so I've tossed him in prison. Most of this is running through the facts I'm aware of. Feel free to add or correct anything.
I ignore the whole future thing. Both Dorian and I come out of that unscathed, and I'm not going to punish someone for events that haven't happened yet.
He kicks out Arl Teagan and indentures the mages. This is bad yes, but not too horrible IMO. By the time we get there nothing bad has yet happened to the mages.
So far the only really terrible thing is the whole mess with the Oculara and the tranquil skulls. Honestly, this is so horrendous and disgusting that it might just be enough, but I still want to know if I'm missing anything else.
Does Dorian have any sort of reaction (as in dialog, not just approval changes) if you either have him do research or execute him? This is one of those cases where I would like to have a follower's input on a judgement. [edit] I just watched all of the judgement options for him on YouTube and Dorian doesn't have any approval reactions to any of them, which I find interesting.