@ Doc Weasel:
If, as the GM, you're willing to let a PC do something that goes not only against the rules as written, but counter to the roleplaying trope of the class, you're stepping onto a slippery slope.
What happens when a wizard wants to wear armor? What happens when a human wants to wear Gromril armor (big no-no BTW)?
Games are played on the unspoken agreement to follow the rules. Admittedly, some rules can be bent, some broken, and some completely thrown away, but I think that you can see here from the voracious disagreement with allowing any slayer career to wear armor, that this is a rule that needs to remain as written.
Consequently, your logic over the Agent losing his employer is flawed. After all, a Ratcatcher that never catches a rat doesn't cease to have a career. A Barber-Surgeon who lances no boils doesn't cease to be a doctor. The Agent is simply unemployed. A Free-Agent, if you will.
And one needn't suffer insanity to be a Witch-Hunter. You can start that career after successfully completing the Zealot career.
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