I have seen it, and known people who have died from it. I also know people who have died from alcohol poisoning, and lung cancer from smoking.
If you were to tally up the number of deaths due to over consumption of alcohol, chronic drinking, drunk driving, fatal violence and accidents due to alcohol compared to heroin, alcohol wins in a landslide.
Now, if heroin were made legal and quality/safety measures were introduced, overdoses would plummet, and most of the crime related to heroin addiction (burglary, petty theft) would also plummet.
The difference between alcohol and heroin is drunk people tend to become aggressive and make terrible decisions by fighting, driving, gambling, etc. The heroin user usually just wants to sit on the sofa. The problem starts when the quality and potency of the supply becomes unpredictable and people OD. If you had a reputable heroin company (Like Bayer once was), OD deaths would be rare.
Not to mention the fact that hundreds of thousands of non violent heroin users and dealers are locked up in prison, which is a travesty, especially for the users. Same goes for marijuana and cocaine etc.
Make drugs legal, tax them, regulate them, take the criminality out of it and watch as societies are uplifted.