Well, probably not, but just so we're clear: if you weren't born in the west, you probably wouldn't be a Christian.
Still, there is a world of difference between your average religious person, and a religious zealot. Religious zealots are more likely to not only harm others, but also to harm themselves. How many stories do we read about high-exposure zealots living closested, disordered lives?
There's also a difference between your average environmentalist, and an environmental extremist -- although I'd like to note that one can arrive at environmentalism regardless of their location, and without any "leap of faith," which makes it fundamentally different from religion.
Still, an environmental extremist is more likely to harm others, or themselves, just like a religious fundamentalist, yes.
And in either case, these are anti-social, paranoid, self-damaging behaviors. Yeah, I'd say we should look into whether it should be classed as a mental illness.
And even more tellingly, there seem to be some people who swing to different kinds of extreme over their lifetime. For example, starting out a religious zealot, and becoming an environmental zealot. There seem to be people who just can't modulate their thinking, and there's even suggestion that genes may contribute to this.
There's also proof that even the majority of relatively normal people can be brainwashed into zealotry. They can literally have their brains re-wired by influence. And yet, some people are invulnerable to this. What makes them different?
I think that certainly warrants investigation.
Oh, they do a lot of things. PETA actually kills most of the animals in their "shelters." About 90%. Some vandalize or even set fire to research facilities. Yeah, they can get pretty bad.