While armed with a knife, if the Waco Kid approaches Martha Johnson and demands then takes Martha's purse and keys, he should be charged with _____.

In this scenario, the Waco Kid should be charged with armed robbery because he used a weapon, specifically a knife, to threaten Martha Johnson and forcefully take her belongings, which include her purse and keys. Armed robbery is defined as the act of taking property from another person through force or intimidation while being armed with a weapon. The presence of the knife escalates the severity of the crime beyond simple theft, as it involves the element of intimidation and the threat of violence.

The act of using a weapon during the commission of a crime is a critical factor that distinguishes armed robbery from theft. Theft typically refers to unlawfully taking someone else's property without the use of force or intimidation. In this case, since there was a direct threat to Martha's safety, the crime is classified as armed robbery. Additionally, vehicular hijacking would apply if the keys or the vehicle itself were taken without the extra context of force while threatening the individual, but the main focus here is the use of the knife and the direct confrontation, making armed robbery the appropriate charge.