What is the difference between alcohol addiction and say heroin addiction?

The substance used.: And the legality of that substance. Any addiction can and does kill, just in different ways.

Addiction=Addiction: Alcohol is not illegal to possess. Alcohol is more toxic biologically. Alcohol withdrawal may be deadly, whereas heroin withdrawal, while uncomfortable, is not life threatening. Heroin, when injected, may lead to various systemic infections. Otherwise, the process of the two diseases may lead in the same direction (insanity and death), and the process of recovery is similar (mutual support).

Same difference: Regardless of the initial neural pathway on which an addictive drug acts, all addictions have a final common effect in the brain - the inappropriate over-stimulation of the reward circuit. The ultimate effects of this final common pathway are the same irrespective of the particular addictive substance used.