He sees Tranquility as a mercy, despite many if not most mages disagreeing. The ones in question are almost certainly doomed either way.
Ok Xil, you have a templar in game saying that mages don't need to be killed just because they are suspected of being blood mages during a ROA and states that it is the duty of a templar to risk sparing a possible blood mage rather than killing an innocent mage and your retort is a refernce to him viewing tranquility as a mercy.
Ok to him he probably would view it as a mercy. He wouldn't want to have magic.
After Hawke's encounter with Ser Alrik in Act 2, Cullen will discuss the merits of the Rite of Tranquility, but will insist that he did not support Alrik's "Tranquil Solution" and give his opinion that the current system of the Harrowing and hunting down maleficarum is the best solution.
In addition he investigates the missing mages and such. I think Cullen was a victim of no one can do anything but the hero plot device.
Oh yes and about his "Mages aren't human beings like you and me," comment for which he is vilified. He is right, Cullen can't use magic so mages aren't human beings like him.
oh and we also have this little tid bit and I am done.
Cullen maintains that the Templar Order's purpose is not only to protect the population from magic, but also to protect the mages that the templars are sworn to oversee.