Depending on which country you're in, heroin can be cut with all kinds of stuff. In Europe for example, the quality of heroin (brown #3 usually) has gone massively downhill lately. I know, I used to be a very occasional user. The last lot I tried was heavily cut with benzodiazepines (confirmed through a home urine test). You think heroin addiction is bad? Try getting off benzodiazepines. It makes opiate withdrawal seem like a walk in the park by comparison.
My point is, like others have said, unless you can be sure of the purity, and have other CNS depressants to combine with the heroin, it likely will not result in a successful overdose and death. Even though I have access to it, it certainly would never be my method of choice.