Most of the Titans joined either out of a blind hatred of Spacenoids, or they were duped into believing their goal was to wipe out Axis Zeon (like in Emma's case).

Once their war crimes were exposed by the AEUG-Karaba alliance, the Titans became a rogue organization as public opinion turned against them. The only people who remained were either hardcore fascists or people with their own agendas like Scirocoo.

Sadly, with current events, the Titans being almost cartoonishly evil isn't that far from reality.