I was using this definition of zealot:
A fanatically committed person.
With this definition of fanatical:
Possessed with or motivated by excessive, irrational zeal.
Admittedly, there are other definitions of a zealot, but that's the one I'm using. By that definition, a moderate person and a zealot are mutually exclusive.
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To further my point, I see an intelligent person who isn't wise as someone who chooses which cause he pursues when contradictions appear between ideals. A wiser person would keep both ideals and find the necessary balance between them. For instance, I see Shojo as being very intelligent, but not very wise. I'd place his alignment firmly in NG. He isn't lawful, but he still prefers it to chaos.
Essentially:
High int characters see that ethics and morality will conflict, but how they deal with this is dependent on their wis score. A high wis will allow for compromise, but a low score will lead the character to choose which he follows, and the other is merely the way his neutrality leans.
Low int characters won't notice the conflict.
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All that said, I wasn't being restrictive. I didn't say, nor did I mean, that an intelligent person would never be an extremist, just that I felt it less likely than the half-neutral views.
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Druids, clerics and paladin variants are the only 'zealot' classes in the SRD. Notice that an int score isn't very useful in any of them, relative to other classes.