That is how I see it anyway. Kingsguard or not, I see Sam as the King's personal hitman.
That's about right. He just killed a man though, so he will be good for a while. He also ain't a Kingsguard, and specificly choose for this because he doesn't want to be stuck to an oath of any kind. He is a free man, and will always be this way.
As for leaving a brutal message, I like that idea.