The only way that "cowardly" can rightly apply is if we consider the murderers had other ways to forward their cause (e.g. politics). Instead of conducting a peaceful revolution, they 'take the easy path', which is 'cowardly'.
But, this is not the only way of understanding the shaping of perception in the public sphere:
This is also media spin for constructing popular opinion. Rightly, I'd add, though I agree that in that regard it's not technically correct to use this word.
Taking a weapon and killing people is in one way not an act of cowering: the murderer (in these cases) will clearly be apprehended and their life will be over (through prison or death sentence). There is a strength (non-cowardice) required to more-or-less end your own life (literally or practically).
The public interest is served by not framing this as any-form-of brave. That would provide praise to the murderer(s). it would probably encouage others.
I have a personal hunch that the usage around around 2001, when media needed a way to condemn suicide bombers for attacks on the WTC. Many suicide 'missions' are performed out of a concept of 'honor'. Publicly-labeling these people as cowards is (perhaps?) the strongest response to in swaying the opinions of those that would 'try next.' This being a more effective response than to use any term that would convey power - even that the murderer(s) had the power to be brutal and savage.