Agreed. After 9/11, the trend became to use any demeaning adjective to describe terrorists. Calling these murderers “cowardly” never made sense to me. However, to objecting to calling them cowardly seemed to imply they were the opposite — meaning, brave — so most people just shrugged off the description.
Maybe it would be better to say: “That cowardly son of a bitch had a lot of balls trying to get away with this.” That way you are acknowledging that he is cowardly for killing defenceless people but at the same time saying that it took a lot of balls to do it.