A) It's absurd to blame the entire race because of the actions of its worst members. We're far from perfect but even with all the violence in the world today, we're at the minimum all time violence per capita. And if you accept Star Trek mythology we keep moving in that direction. After nearly wiping ourselves out, of course.
B) Even if you can sympathize with the changelings' reasons for being afraid of solids you certainly can't sympathize with their actions. Just like having a traumatic childhood doesn't excuse you for beating your wife. You're responsible for your own actions toward sentient creatures, period. Invading the alpha quadrant wasn't even in their best interest. They could have gotten themselves wiped out, and if there was any time for a gamma quadrant rebellion, it would have been when all their warships were in the alpha quadrant.