Well said, Aims.
People don't choose to get addicted and, as we know, they're often the last to admit they are.
They start because they can't cope with something and often feel they have no-one to turn to who can help them cope. A drug makes them feel better, just for a short time. Then they come down and need the high once more - again, because it's often the only way to blot out the pain of whatever's happening. Only the next time, they need a stronger dose in order to get the same feeling.
Eventually, even the strongest oral, smoked or snorted dose doesn't work well enough because, if you like, the user has become immune to the effects. That's when they begin to inject. They can't give up by now. 'Cold turkey' withdrawal is physically painful and is often horrendous to witness, much less experience, I would imagine.
We can't help many of those who get hooked, because they've become so brain-addled that they think they don't need help. Of those who do accept help, they often aren't strong enough to stay clean and fall off the wagon time and again. It doesn't make them bad. It doesn't make them mad. They are simply people whose weakness was taken advantage of and then became ill - because that's what stuff like heroin does - it makes you ill, in the mind and in the body.
Going on from what Aims said, those who kick the habit, I believe, are given too little support afterwards. More support goes to time-served muggers and child-molesters than goes to ex-addicts, and that is far from fair.