I would call our reason pragmatism.
In Braavos we wanted to talk, but he foresaw a bad ending and fled.
In Pentos we didn't dare to face him fully prepared and weren't capable of good and quick capture, so we killed him.
After that his mind was twisted by a dark god and even without that we killed him, so we didn't want to trust him after that matter.
This is honestly true, and even thinking back to when I was only reading the quest still, I wondered if perhaps we would have simply just cut a deal with him instead. It would have been a free mage with ambition and a reason to stick close to us, and we could hardly afford to turn back the extra hands and spells back then.
But the vote was either interpreted as too likely to become a violent confrontation (which either suggests that DP weighed some of the discussion that he had read as opposed to hadn't at the time more heavily than what was had) or that the man already was unstable and paranoid, and it just grew worse and worse and we would have been escorting a ticking time bomb waiting for the right chance to stab us in the back.
@DragonParadox You don't do it anymore, which is really nice, but you made a lot of the people who eventually became our enemies filthy-mc-bad-bad in a form of hindsight bias. We never seemed to discover anyone who did even one greyish or amoral act to have been hiding a proclivity toward feeding starving orphans, for example, they were either your run of the mill pack of barely-not-bandits like the mercenaries we slaughtered, or worse, were actively cruel and malicious by nature.
So I would say it's just likely Tor was written this way. It would have taken a lot more thorough intervention to get Tor to stop being
insane for example, intervention which would have come with limiting his actions and abilities, which would have set off his paranoia, so...
Yeah, that's why I wasn't looking forward to this discussion, we might have acted mostly out of simple pragmatism and multiple years not being able to afford to take chances (whereas now we
can take more chances because we're secure in our power).
I wonder if any of them actually notice that there are marked differences in what we can afford to do now, as opposed to back then, though?