I have had fur fall out from the areas where I applied flea control on my cats as well. That's why they tell you to use a small amount as a "test dose" first.
You are correct that most of the reviews on this product are not very favorable---the product doesn't work. That said, it is still unlikely that peppermint oil, cloves, and lemongrass killed your beloved kitty.
You must look at EVERYTHING in your household. Commercial pest control is very toxic to pets. Cleaning agents, laundry detergents, hair spray, the food you feed your pets. What did kitty eat? Was it commercial pet food? There are a few books to look at regarding that issue. Ann Martin has written "Food Pets Die For", and it is eye opening. I was feeding high-end commercial dog food to my dogs, only to find out that it contained contaminated ingredients, and that there was virtually no way of knowing for sure what REALLY went into that bag of dog food. Most pet food manufacturers have a "co-manufacturer" that makes the food. There is little or no communication between the two. When I discovered I had been feeding contaminated food to my pets, the manufacturer blamed it on the co-manufacturer, stating thay had added an extra ingedient the the primary manufacturer was not aware of. That ingredient was not even listed on the label.
You must look at EVERYTHING. The August/September issue of Animal Wellness Magazine had a list of common toxic to pets chemicals that almost all of us have in our homes. Check it out and see what you think.