On sight works for me, if the person is on video committing the acts.
Just line them up and shoot the legs out first, then hands and arms, the stomach area, then the chest followed in a minute or so by the head.
Make it as grisly as you can, and televise it.
Do it with a clearly visible easy to read question that reads?
Wanna be next?
Anyone that objects is free to line up with the perp and share their fate as far as I am concerned.
Amen.
In this era you also need enough due process to rule out the possibility of computer faked video framing an alleged perp. Although in many mass shootings there is overwhelming eye witness evidence with perp literally caught in the act.