It is an empirical question whether the cowardly terminology will influence these folks.
I do think the coverage affects them but it is more in the vicarious reward of seeing the pain they cause. Supposedly, they have a mission to target population group X. This is correlated with a suicidal wish and to die a death in seems what is combat to them. A warrior's death, so to speak - either shot by the law or a noble suicide. They see the pain and uproar of a previous rampage and it rewards them as they think about their own plans.
So when, Dad and Mom Tommy Football are seen as crying and wailing when their hero son is dead and Dad calls the killer a coward - what is the impact on the next guy? The grief and pain rewards him more than the insult deters him. He or she would regard that as just a utterance to be expected.
It is thought that cutting down the coverage is more important than ranting about the killer. Seeing the pain is what they want. Or so we think about the motivations of some.
So I heard that Holmes wants to plead guilty to avoid the death penalty and get life. I would take that deal and get him off the front pages as compared to a trial. Same with Hassan - don't want a show trial for the next nut to fantasize about. The military court, IIRC, turned down a guilty plea. That's stupid - take it and get them out of the media circus.
The social impact on them, IMHO, isn't in the classification but in the grief they cause and we replay.