Apparently the wood chipper is an alleged torture device of Saddam Hussein. There doesn’t appear to be any proof of its existence. From what I’ve read it was more of a fabrication of propaganda used by the west.
We know how much it takes of anesthetic agents, potassium, etc. to kill a person, and can standardize the dose. That’s not the case with heroin.
In addition, it’s not going to kill the person right away. Those stories about a rock star being found dead with a needle in his arm? He didn’t die from an overdose; he had an anaphylactic reaction to something the dope was cut with.