Tim Bowen sheds some light on the origins and definition of the phrase to be caught red-handed.
If you are caught red-handed you are discovered in the middle of committing a crime or doing something wrong. Criminals can be caught red-handed by the police at the scene of their crimes and children can be caught red-handed by their parents committing some minor misdemeanour like taking food from the fridge in the middle of the night.
For many centuries it was not considered to be proof of guilt if someone was in possession of meat from another man’s animal. It was impossible to prove that they were guilty under such circumstances. If, on the other hand, they had the blood of a freshly killed animal on their hands this was incontrovertible proof of guilt. In those days being caught red-handed usually meant a death sentence. Today, while the punishment is likely to be far less draconian, the idea of being discovered in the middle of wrong-doing still remains with this expression.