Actually it's the abuse of drugs, impure drugs and drug inter-reactions that kill people. That's why legal drugs have to be prescribed by medical professionals (i.e. GP's, Doctors, Nurse Prescribers) who generally know what drugs are suitable for a given patient and their condition, plus also in what combination they can be given to reduce the risks of drug interaction. Not only that, but many prescription medications can have potentially lethal side effects depending on the person taking it. Just read any leaflet for an everyday medication to see what side effects can happen with so called 'safe' and accepted drugs that we take for granted. Now multiply those risks many times for a recreational drug bought off the street, concocted in a backstreet room with no quality control and with questionable and impure properties. You may get lucky and get a 'pure' shot of your favourite substance so won't risk being poisoned by whatever a dodgy dealer chose to cut it with that day. Or maybe you weren't expecting it to be so pure and take your 'normal' dose only to find that because it's so pure you've taken a massive overdose. Maybe it will react with a prescription medication you're already taking, because the dealer who sold it to you hasn't a clue regarding drug compatibilities and can't warn you about this.
Good luck to those that still choose to ignore the warnings. It's your life and your choice.