1] How many do you think with in that environmental group had any clue about what MacFinn was? ...
I doubt it was many more than Kim, unless she worked with a coven/etc and told
them; maybe an "eco-coven" or something?
But that's entirely unattested in the novels (or WoJ) that I know of.
I suspect she -- as a magician of SOME sort -- had an inside track of figuring out that MacFinn dwelt on the spooky side of the fence himself, and SHE approached HIM (or did some scrying / etc to figure it out).
2] If you had a clue as to what kind of damage it could do to the organization, if word got out publicity wise. If the press found out about MacFinn, then what? Just another thought to think about. Did Kim know what Harry's girl friend did for a living? Further did she know what paper Susan worked for? If she did, there is your motive for withholding vital information from Harry and outright lying to him ...
Or just the "Supernatural SOP:"
maintain the masquerade. It's not zealotry if it's what
everyone (who's a spooky-sider) is doing!
Not to mention that, as a supernatural herself, she could expect to be harassed or even attacked by "suffer not a witch to live" fundamentalists; that puts her on MacFinn's side, no zealotry needed!
I don't doubt her sympathies were invoked: poor MacFinn, victim of this awful curse on his ancestor, etc etc etc. He had managed to work out a way not to kill people, he was just trying to help the world and not harm anyone, won't you help me please?
On the flipside (the "stick" if you consider the points above "carrots") Kim could reasonably expect to be locked up as insane, or sued into poverty, if she turned on him & tried to unmask MacFinn -- noted socialite / financier / whatever (where DID his wealth come from?) and philathropist; general Good Guy of no small repute -- as a freaking
werewolf Gimme a break, lady!
Not seeing zealotry. Seeing an attempt to keep a status quo (the Masquerade), to help a respected elder & asset, some personal sympathies, and a backstop of "more likely to get in trouble than to
fix anything, if I try to turn informant."
3] In the face of all the warnings, not getting all the information to build the circle, still keeping his secret and attempting something she knew she couldn't handle at the cost of your own life ...
Or simple overconfidence.
Seriously -- the "zealot" attempting something they know is going to fail??
Maybe the jihadist "dying in the right cause guarantees heaven" mindset will
intentionally "die trying" (with no expectation of success).
I don't think we have ANY evidence that Kim was in that sort of mindset.
Instead, I think she seriously hoped she COULD do it. She knew the consequences of failure -- not just that she might die, personally; but that her beloved cause would be WORSE OFF. Consequently, she would have been
expecting to succeed... or at least, seeing this course as the
best odds of success.