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.