In any case, I can't agree with anything you said. Obviously reducing murders by half will have very deep trickling positive effects on a society. And the Salvadoran Civil War was simply not due, even in part, to cracking down on gang crime.
I reiterate that, even though you may not like it, or may want to use softer methods, or may resent one for making this observation, repressive methods provably work at stopping violent crime. Unless you're a slippery slope afficionado, what this should tell you is not that you should declare martial law on Earth, but that violent crime is a provably solvable problem. Arguably, alongside with morals, it would be nice to also have the imagination and motivation to achieve such result.
I guess we'll see how stable El Salvador seems in a few years. I would bet that it will either be an authoritarian state rife with human rights abuses as a matter of daily operation or it will be once again overrun with crime bordering on civil war.
El Salvador is very likely committing extrajudicial killings and imprisoning people without anything resembling fair trials. Again, this will have the intended effect in the short term and will very likely not in the long term.
For example, why does MS-13 et al exist anyway? Well, as consequence of the Salvadoran Civil War which was kicked off in part due to extrajudicial killings that the government at that time was using to maintain control over a brittle society!
You’re just seeing a repeat of the same cycle that produced the violent gangs in the first place. The problem will re-emerge and I’m sure we’ll have another proposal to do “the simple thing” and just go imprison and execute a bunch of people extrajudicially. Maybe it’ll work that time.