Is it murder if you kill someone to save the life of a police officer?

ok say you were out hunting or something and you were in a city or town. and you had ur rifle in your vehicle and saw a intense firefight between a cop and a bunch of gang members or whatever. and you pulled your rifle out and killed those people would it be considered murder? in essence you saved the police officers life. or would there be some other kinda violation you commited? what would actual happen. from the legal standpoint