If you want to understand why people would take a drug that makes your flesh rot, just look at this guy's face again:
That, my friends, is pure, unadulterated despair. That is a man who has lived a horrible life and who knows that he will continue to live a horrible life, and is just hoping to numb the pain any way he can for as long as he can, a man for whom death is preferable to living without the drugs to buffer him from the world he exists in.
If you want to understand why people take this shit, look at where it first emerged. I mean codeine is just as easily available in most countries (USA is the exception in making it prescription-only), but the difference is that Russia has a) fucked up quality of life for a huge number of people, not many prospects, a lot of poverty with not a lot of help, and b) absolutely no attempt to help addicts, no needle exchanges, very few rehab beds (and those are in very harsh places, little better than prisons) and no methadone or buprenorphine opioid replacement therapy, c) a lot of heroin, but not, and this is important, a lot of cheap, affordable heroin.
These aren't kids who were bored one day and said 'I wanna try a drug that turns me into a zombie.' These are kids who had any hope of happiness ground out of them at a very early age, who turned to heroin because it was there and because it made them not care about how fucked up their life was, who ran out of the means to continue their habit but couldn't deal with the agony of withdrawal, let alone a return to the exact same sober life that had driven them to heroin in the first place, and reached a point where all they cared about was keeping that blanket of numb warmth firmly wrapped around them, at first there would be a lot of 'I'll only do it this once, to get high again for a few hours, surely once won't rot my arm off, and I'll find a way to afford some heroin in the morning,' involved, it wouldn't be an entirely conscious process, but eventually I'd imagine they just stop giving a fuck, as long as they can keep their high going and ignore the cold reality around them for another day.