The Commonwealth is not required to prove that the defendant himself performed the act that caused the victim's death.21 However, to establish that a defendant is guilty of murder [or voluntary manslaughter or involuntary manslaughter], the Commonwealth must prove two things beyond a reasonable doubt.
First, the Commonwealth must prove that the defendant knowingly participated in the commission of the crime [identify the crime if needed to avoid confusion].
Second, the Commonwealth must prove that he did so with the intent required to commit the crime.22
Such knowing participation by the defendant may take many forms. It may take the form of personally committing the acts that constitute the crime, or of aiding or assisting another in those acts.23 It may take the form of the defendant asking or encouraging another person to commit the crime, or of helping to plan the commission of the crime.24 Alternatively, it may take the form of the defendant agreeing to stand by at, or near, the scene of the crime to act as a lookout, or to provide aid or assistance in committing the crime, or in escaping, if such help becomes necessary.25 An agreement to help if needed does not need to be made through a formal or explicit written or oral advance plan or agreement; it is enough if the defendant and at least one other person consciously acted together before or during the crime with the intent of making the crime succeed.26
The Commonwealth must also prove beyond a reasonable doubt that, at the time the defendant knowingly participated in the crime,
[identify the crime if needed to avoid confusion], he possessed or shared the intent required for that crime.27 28 You are permitted, but not required, to infer the defendant's mental state or intent from his knowledge of the circumstances or any subsequent participation in the crime.29 The inferences you draw must be reasonable, and you may rely on your experience and common sense in determining the defendant's knowledge and intent.30
Mere knowledge that a crime is to be committed is not sufficient to convict the defendant.31 The Commonwealth must also prove more than mere association with the perpetrator of the crime, either before or after its commission.32 It must also prove more than a failure to take appropriate steps to prevent the commission of the crime.33
Mere presence at the scene of the crime is not enough to find a defendant guilty. Presence alone does not establish a defendant's knowing participation in the crime, even if a person knew about the intended crime in advance and took no steps to prevent it. To find a defendant guilty, there must be proof that the defendant intentionally participated in some fashion in committing that particular crime and that he had or shared the intent required to commit the crime. It is not enough to show that the defendant simply was present when the crime was committed or that he knew about it in advance.34
Where an element of an offense is that a person who committed the crime possessed, carried, or used a weapon, the Commonwealth must prove beyond a reasonable doubt either that the defendant himself possessed a weapon, or that the defendant knew that a person with whom he participated in the commission of the crime was armed with a weapon.35 However, mere knowledge that a participant in the crime was armed is not sufficient to hold the defendant liable for the acts of that participant. The Commonwealth must also prove that the defendant knowingly participated in the commission of the crime, with the intent required to commit the crime.36