Technically, though, he never said Mages aren't people. He said 'mages aren't people like you and me'. I'm not about to defend the harsh language he used, but he IS right and did have reason to think this.
(For the sake of this, let's ignore Mage!Hawke for a second. This isn't about 'lol, he's so unobservant'.)
Mages can be very good people. Mages can also be very bad people. That's human nature. However all mages are, whether you like it or not, people that are in danger of being possessed at every waking moment. They're people who have a LOT of power at their finger tips. So they are technically living weapons that can have their control taken at any moment if they're not careful. It's easy to say they have the will to resist, but, everyone has their emotional low moments. Everyone has losses in temper. Everyone has moments where their mental walls are weak.
Some of the fear of this is lost on the player, though, because the player character is never in danger of this part of the lore.