Agreed, he probably would not do it. Massive slaughter isn't generally profitable.
True, but that's only an issue if it's /human/ slaughter. I maintain that Chitzen Itza would make the perfect place for a Darkhallow but would be sucking dense Life of an entire Jungle ecosystem rather than just that of the human (and presumably pet & vermin) populations of an Urban jungle. It would be a Quality over Quantity thing on the Life Sucking side (assuming Zombie necro rules apply), and on the Spirit side they have all those sacrificial Dead from
millennia of Bloodsport and vampiric rule; waaaay better than just some generic native American Hunter spirits.
Where do you get that he would lose freedom of Choice? Ascension to immortality is akin to taking on a Mantle, but we haven't been given any info on what kind of obligations the Darkhallow would bestow. Now, sure, Immortals are under a lot more scrutiny by the powers that be, but that isn't removal of choice. It's the practical restraint of action based on presented consequences.
The strong implication from a bunch of WOJ's is that Free Will (in the cosmic superpower sense) is exclusive to Mortals, and thus to become truly Immortal (in the Mantle Sense) is to loose that soul. We know that is guaranteed (though not instantaneous) for Molly, for example.
The best theory/framework Ive come up with is that Mantles are solidified True Names (where Names are themselves an aspect of The Self along side the Body, Spirit, Soul, etc) that have gained enough energy and/or momentum to outlast their original composite form, not unlike how a Spirit-ghost can be described as /Memories/ that have gained enough energy to exist beyond their original Self. This Solidified power is incompatible with whatever fluid Soulness it is that lets Mortals Change their Names and Themselves and The Universe Itself via Choice. FWIW, it was also theorized that part of the Normal Soul function of a person relies on that thing Bob talks about where Soul chunks get passed to loved ones through Hugs and whatnot; the idea is that normal mortals trade and regenerate Soul regularly, but becoming a Mantled Immortal would freeze the human Host's ability to regenerate their own Soul. So they'd be left with a Finite supply rather than the renewable resource that Bob described to harry. So if she wanted to keep her soul Longer, she needs to maintain as many Real human relationships (family, friends, etc) as possible so that she can get a regular influx of Soul through those interactions. Thus, to loose her connection to her Mortal Life would be to LITERALLY loose her soul.