Murder and Premeditation

Under Elizabethan law, first-degree murder involved Malice Prepens (premeditation). The planning of the murder is the operative factor, not the way the plans work out. Thus, if a person planned to kill one victim, but murders another (say by accident), the premeditation still makes it equally odious.

It is not clear that this consideration applies to Hamlet's murder of Polonius. Of course Hamlet imagined that it was Claudius behind the arras, and not Polonius, but he was acting on the spur of the moment. This was not a planned murder.

On the other hand, the murder of Gertrude fits this consideration precisely. The poison was set for Hamlet; the murder planned out. According to the law (if the case had come to law), Claudius could not claim that Gertrude's death was accidental. It was a planned, and thus premeditated, regardless of the blunder, and would be treated as such by the courts. Claudius was guilty of the first degree murder of Gertrude.