The military industrial complex is a reference to the entirety of the corporations and their industrial factories they created to make weapons of war and destruction and protective gear and deterrents or vehicles to drive and deliver those weapons and gear.

It refers to the buildings the work force and the people that profit, even, technically from selling weapons and protective gear related to fighting and war.

There's nothing that merits human centipede. Just kill them if they're unrepentant, there's mountains of evidence and you know they'd just offend again if they got out.

A convicted killer could be someone that killed in defense of another or themselves that was blamed for a crime they didn't commit.

You also don't know if a convicted killer was on drugs or had a mental illness.

Manslaughter can be an accident.

What purpose would it serve?

The point of incarceration is rehabilitation, not gratuitous torture to appease revenge boners.

This would just point out that the purpose of incarceration is for sadistic entertainment.