I had pretty bad drug problems when I was in college, causing problems that have persisted until today. I saw some fucked up things. Talking about opiates in particular, which was what I got involved in, I can say that it becomes an issue of staying "well". Your body becomes dependent on the drug to function, and when the drug wears off, you feel like you have the flu and get depressed and anxious. It really sucks, and that's what drives people to stealing and ripping people off.
One of the craziest things that happened during this time for me was a heroin deal that went south. I was in the car with a "friend" of mine and someone that needed H called him saying that he scrounged up some cash. My "friend" did not have any H, but wanted to get the money off of this guy, so he lied and said he did have some. We stopped at a convenient store and he punctured a bag of cat litter and took out a couple of the rocks, and we bagged that up and made it look like heroin.
Now, it's not particularly hard to tell the difference between cat litter and heroin, but oftentimes the heroin is cut with something that makes it less pure, and this guy that bought the kat litter was desperate, and figured he'd go ahead and try shooting it up anyways, knowing that it probably wasn't heroin, but there was a small chance it was. A few moments after the injection it became clear that there was no heroin in the shot, and my "friend" played it off saying that he must have been ripped off himself because he paid good money for it.
The guy that took the shot started vomiting and we had to take him to the hospital where he spent several days.
Really fucked up story, I know. Almost killing someone just to get like 60 bucks to go and buy a bag.