^ All good and well in theory.
Of course an invasion and killing all of the bastards will work. We have seen this in Germany after WW2, when all the former elites were instantly re-installed, to get at least a bit of control over a wrecked country. 'De-Nazification', my donkey. This was the reason for the '68 student revolts, when the young found out about reality.
Singapore is a very bad example, Asians are way more easily brought under control by draconic measures, while Mexicans will more probably revolt. Of course "you" can shoot them all.
What you and a lot more seem to completely misunderstand is where the root of the problem is. Even shooting them all will not cure the problem, only the symptom – if at all.
The US agency DEA fights a decades long war against the drug cartels, against corrupt politicians and against the Mafia. And both are situated in.. the US.
Another problem is the CIA. When Reagan (ab)used the CIA to deal with drugs and finance some of
his invasions, the DEA, while having a hard time doing so, exposed all of those lies and drug deals. This information was not directly suppressed, it was just that nobody wanted to hear about it.
The genereal philosophy is like
"There is always going to be a drug trade and a drug cartel, it might as well be our drug cartel."
If you invade Mexico and "kill all", Columbia will gladly get into business. And then there is Afghanistan. And so on.
There is a heroine epidemic in the USA, but also in parts of Europe. As long as this persists, there is no easy final solution.
Some more info on one of the drug lords:
https://www.thedailybeast.com/don-wi...about-el-chapo