haha .. that's cute about layla not knowing you.  
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I think the coward debate depends on your relation to the terrorist.  If you were standing on London Bridge one day (or had just left an Ariana Grande concert) and were mowed down from behind by a car or blown up by a bomb, you would absolutely think the attacker was a coward.  
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It's a more violent form of a sucker punch (there was a game at one point called 'knockout' where people punched unsuspecting strangers in the face).  That is absolutely cowardly.  Even though if done incorrectly the assailant could face retaliation.
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If you are from a Muslim culture you might explain the actions as a retaliation against a larger aggressor and justify not facing their army head on ... but that is simply a justification for an otherwise cowardly act.