I'm sorry for your loss. You must be devastated.
But you have to keep a few things in mind;
I don't think the brand has anything to do with your cat dying, but the active ingredient which caused a reaction with your cat. Any other collar with Tetrachlorvinphos as its main ingredient could have done the same thing. Unless the collar was defective, and there was too much or something.. Maybe you should have it tested?
Also: Anyone with a keyboard and an internet connection can create sites to promote their point of view, and make it look very convincing. I came across the same sort of sites you probably did (like HartzVictims.com) and it looks legit, but just remember, it could have been a freak mistake, or maybe not even related to the collar.
Not that I have any reason to defend Hartz, but I like to see more logical thinking on forums (is that too much to ask?
)