How did the jury know the rules that spelled out the meaning homicide? This is about the Boston massacre btw

The Jurors for the said Lord the King upon oath present that Thomas Preston, Esq.; William Wemms, laborer; James Hartegan, laborer; William McCauley, laborer; Hugh White, laborer; Matthew Killroy, laborer; William Warren, laborer; John Carroll, laborer and Hugh Montgomery, laborer, all now resident in Boston in the County of Suffolk, ... not having the fear of God before their eyes, but being moved and seduced by the instigation of the devil and their own wicked hearts, did on the 5th day of this instant March, at Boston aforesaid within the county aforesaid iuith force and arms feloniously, willfully and of their malice aforethought assault one Crispus Attacks, then and there being in the peace of God and of the said Lord the King and that the said William Warren, with a certain handgun of the value of 20 shillings, which he the said William Warren then and there held in both his hands charged with gunpowder and two leaden bullets, then and there feloniously, wilfully and of his malice aforethought, did shoot off and discharge at and against the said Crispus Attucks, and that the said William Warren, with the leaden bullets as aforesaid out of the said handgun then and there by force of the said gunpowder so shot off and discharged as aforesaid did then and there feloniously, willfully and of his malice aforethought, strike, penetrate and wound the said Crispus Attucks in and upon the right breast a little below the right pap of him the said Crispus and in and upon the left breast a little below the left pap ... of which said mortal wounds the said Crispus Attucks then and there instantly died.