The way I see them, I found Team Plasma to be modeled after real life extremist groups that have ends that justify their means even if it means being hypocritical. Like the groups that bomb scientists' cars because they don't like the scientists experimenting on primates- protesting violence against primates by using violence against other primates (us humans).
I assume a lot of members of Team Plasma truly did believe in Pokemon Liberation, but to reach that goal they needed to first use their own Pokemon. And then when they finally created a world where Pokemon can all live free from humans, they can set their own Pokemon free. It's the lesser of two evils in their eyes: even though their Pokemon are being held captive, they are being used to try and free those captive Pokemon that are constantly forced to participate in combat for the entertainment of their masters. To Team Plasma, at least their fighting was for a noble goal and not just for their own entertainment.
Of course, I'm sure there are members that are interested in it purely for the power and to be a part of an organized social group (with all of the benefits that can come with that). And these were the kinds of Plasma members fought in the second game, like Sopheria mentioned.
