Bob Ewell is such a despicable character in Harper Lee's novel, To Kill a Mockingbird, that cowardism is only one of his worst traits. In addition to the poor treatment of his own family, Bob concocts the rape and assault story out of pure racial hatred. He has no doubt beaten his daughter, Mayella, before, and no one in Maycomb would have concerned themselves with the physical abuse that he dealt her on the day Tom Robinson came to "bust up this chiffarobe." But, Bob's hatred of blacks--really, all humanity--runs so deep that he sees the chance to punish someone that he considers even more indecent than himself. After beating and choking Mayella, he coerces her to follow his lead and back up his story that Tom haw been the attacker--and a rapist to boot.
Of course, Bob doesn't stop there. His day of glory on the witness stand is muddied by Atticus Finch's staunch defense, and both of the Ewells are subsequently ridiculed by their own inept tale of lies. Tom Robinson is found guilty because he is black, and a black man's word will never be accepted over a white man's in 1930s Maycomb--not even a Ewell's. Nevertheless, instead of earning the sympathy of the town, Bob simply lowers himself to new depths.
Of course, Bob Ewell's cowardice doesn't end there. He spits in Atticus's face and challenges him to a fight--one he will not win against even the aging Atticus. He stalks and frightens Tom's widow until he is threatened himself by Link Deas; as most cowards would do, he backs down. The height of Bob's cowardly ways comes that Halloween, when he attempts to make good his threat against Atticus by killing his children.
Liar, racist, stalker, batterer, bushwacker, and possibly even incestuous, all of these traits add up to a coward of the worst sort.