They are so desperate for a fix that they can't help themselves.  They don't 'want' to take poison - they 'have' to.  This is called being addicted.  I think if you had ever met someone in this position you would see that they are not doing it because it is fun - they are desperate.  

The measure of a society is how well it treats its weakest members, and these are surely some of those who are most in need of our help.