Demand to stop the government from getting away with murder!

So I was watching a TV show called "The First 48" and a 17 year old got a death sentence, therefore he got killed, for stabbing 3 people, my perspective being that murder is against the government, and the government is killing people when they give a human a death sentence, and they are committing a crime, it's against the law, but nothing happens to them when they kill someone(the death sentence).