You're jumping all over the place here. Walmart? Portugal? Assassination attempts?

I suggest looking into the ways different states and counties in the US manage and regulate the sale of alcohol and marijuana to get a realistic idea of what legalized drug sales could look like.

Portugal decriminalized, not legalized, which is a gargantuan difference. Decriminalization is basically the government saying "we'll stop enforcing drug laws, thereby de facto endorsing violent criminal gangs to handle the sale and manufacture as they see fit". As a result, drugs don't get less contaminated and black markets continue to thrive. I think it's a cowardly half-step.

And if the US legalized drugs, who are the cartels going to assassinate exactly? US Senators? The President? Then there actually might be a US military intervention and I doubt the cartels want that.