For he did put his life in his hand, and slew the Philistine, and the LORD wrought a great salvation for all Israel: thou sawest it, and didst rejoice: wherefore then wilt thou sin against innocent blood, to slay David without a cause?
He risked his life to kill the Philistine and God saved everyone. You saw this and were happy. So why would you let someone wrongly kill David, who is innocent?