The First Murderer has some grievances against Macbeth that the latter convinces him were in fact Banquo’s fault.
He has had a hard enough life that he is willing to turn murderer if it gives him a chance at fortune, as he does not much mind dying at this stage. He welcomes the Third Murderer to their party. He strikes out the torch carried by Banquo and Fleance to make killing them easier, but allowing Fleance to escape. He slices Banquo’s throat to be certain of his death. He later takes part in the sack of Macduff’s castle, killing his wife and children.