Loads of clubbers take Ecstasy tablets and seem to come to no harm, as you say Jack. Among the drug users I spoke to, the substances which caused aggression were speed and cocaine. Opiates and cannabis gave rise to torpor and demotivation. Ecstasy merely induces euphoria,
although in some cases it can cause overheating of the body and resultant dangerous dehydration.
I believe some people (and as keberoxu says, it does seem to be genetic) are predisposed to addictions, whether it be alcohol, food, drugs or other substances. I seem to remember an article in which scientists claimed to have found such a gene among quite a significant proportion of people of Irish origin. But of course, social norms and cultures also play their part.