Probably a cocktail of MDMA, methamphetamine and other designer drugs that are cheaper, because that's what you get on an illegal and unregulated market. MDMA can still cause death, psychological issues, seizure and decrease cognitive function. It very well could have been concentrated MDMA that they were taking and not used to.
The fact is that if a drug isn't regulated and producers aren't tested you will never know exactly what you are getting (unless you are a chemist and can test that, but it would destroy your drug.) I have never done molly but I went to college with plenty of people that would swear by their molly being pure MDMA and that it was better than ecstasy because pure molly wasn't bad for you (which is false.) I mean all they did was listen to what their dealer told them. Who the hell knows if they were A: telling the truth, B: Lieing to you, or C: getting lied to themselves.
Its not unheard of for people to mix cocaine or other more addictive drugs in with the weed that they sell to new customers to get them addicted to their product (and slowly phase it out to just mj) without them knowing just so that a person will be hooked and keep buying from them. So if you're going to be doing these things be careful.
The "safest" drugs that you can do are the ones you're prescribed or the ones that you can buy on the legitimate market. That means move to Colorado or Washington.