This is term used when someone is caught doing something wrong. The person doing crime is caught while doing that crime and cant say anything in his defense.
I am assuming “red handed” means hands full of blood. Which apparently means The person caught has his hands red with blood implying he caused that blood and he is the murderer.** I think this reasoning is wrong. A person cud have hands and clothes full of blood while he is standing next to a dead body but this doesn’t necessarily mean the “red handed” person is murderer.**
If the base case isnt rite then why do we use this proverb in our daily while when we catch the criminal on the spot doing the crime.