Zealots have gained a bad reputation by critics who (rightfully so) often pick out the seeming unchecked hypocrisy in the zealot’s behavior.
The zealous critic, however, will often ascribe outward ends as the cause behind the zealot’s zeal and paint him as a fanatic (especially in matters that seem trivial to the critic.)
What if some (maybe most?) zealots actually believe what they’re zealous about? What if they do think it will bring about a better world, or solve some great problem?
Even if the cause has great flaws which the zealot misses, to miscast him as a fanatic out for some other end fills the critic with a misguided zeal–some might even say he has become a zealot in the cause of ending all “zealotry.”
Attacking the person and attempting to mind-read are losing arguments. Splitting the person and their belief from what you think they must be out to do will change the way you perceive everyone around you. It serves to think better of others, argue in a more productive manner, and be more humble in your own conduct.