Legalisation has pros and cons. Good quality gear would obviously make it safer to use but heroin adiction is a bit different to something like smoking cigarettes. A heroin adiction can be expensive and legal or not lots of people will still resort to crime to pay for it. I know adicts that work in good jobs and can aford their habit but most I know are thieving bastards that will pretty much do anything for their next fix. Legal heroin won't change that.
I also don't think it would put cartels out of buisiness like people seem to think, it might affect what they charge but they wouldn't stop.