“Cruel and unusual punishment” applies here.

Government can’t do that constitutionally.

That leaves lynch mobs, which violates right to a trial by jury of their peers.

That said, lethal injections and their “they won’t feel a thing, they just go to sleep” should perhaps be moved away from. There needs to be a degree of fear and pain involved to provide the example, not just “you were very bad, now go to sleep little baby”. Hanging or a firing squad both seem fine.

Part of society should include having a healthy fear of egregiously trespassing on your fellow man.