Usually the people selling these to individuals don’t know what it actually contains. They just buy it from higher up in the chain assuming it is what they say it is.

The people who do make these pills will add fentanyl for multiple reasons but none of those reasons are to kill the user. It’s because fentanyl is cheap to make and a lot more powerful. You can smuggle a much smaller physical amount of fentanyl than something like heroin. Because of that, they’ll smuggle less of another drug and make up for the difference by adding fentanyl. The intention is never to add too much of it but they make careless mistakes and end up with some pills containing a lethal amount.

Besides the issues caused by dealers adding adulterants and impacting the purity of drugs that way, because illicit drugs aren’t professionally tested and produced in less than optimal conditions it’s possible that drug labs will produce at varying purity levels or make something completely different to what they intended, even if they’ve been successful hundreds of times before.

From Wikipedia: “In 1976, a 23-year-old graduate student in chemistry named Barry Kidston was searching for a way to make a legal recreational drug… Kidston successfully synthesized and used desmethylprodine for several months, after which he suddenly came down with the symptoms of Parkinson’s disease and was hospitalized. Physicians were perplexed, since Parkinson’s disease would be a great rarity in someone so young, but L-dopa, the standard drug for Parkinson’s, relieved his symptoms. L-dopa is a precursor for dopamine, the neurotransmitter whose lack produces Parkinson’s symptoms. It was later found that his development of Parkinson’s was due to a common impurity in the synthesis of MPPP called MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), a neurotoxin that specifically targets dopamine producing neurons.” en.m.wikipedia.org/wiki/Desmethylprodine