They are really just normal people doing their job. They believe that they are protecting the peace on the Ark and will carry out their orders to do it. In one gameplay video their commander gave orders to not shoot unless shot at and in another the security leader showed little joy about going in to rescue a hostage from the resistance. He wanted to get in and get out with as little fighting as was possible.Corporal Yakob said:Oh.....what are they like?Tibs said:They are not like that at all actually.Corporal Yakob said:I'm assuming Secruity are the ruthless, heavy handed gas-mask wearing Fascist government forces? Sign me up!
One of the key points in Brink is that neither the resistance or security are bad people. There is no good and evil. One side is fighting for order(unfair as it may be) and the other is fighting for more resource distrubition amongst themselves believing the founders are holding out on them.
The Ark is designed to hold 5000 people, not tens of thousands. There are not enough resources to go around for everyone and the founders are aware of that and acted as most humans would, in the best interest of their peoples. They give what they can to the resistance, but its not enough for them.
The resistance sees the security as unfair and tyranical and the security sees the resistance as terrorists fighting for anarchy. The lines of good and evil in this game are for you to decide.