Wow that's a distressing image. I only have ducks but they are very social and peaceful creatures. They do walk toward my very skittish barn cat, out of curiosity I think, but not aggression. Plus ducks do not have teeth or claws, so I think that limits the damage they can do. Not that they cannot do damage, but before I attributed a death to a duck, I would evaluate the observable injuries on the deceased. Also, the injuries that I have seen inflicted by ducks generally start out small, with potential to get worse. Meaning two things: 1) takes time, and 2) therefore, I as the operator of this farm have the opportunity to get in there and intervene and stop the situation from escalating and injuries getting worse.
And I agree absolutely than mean and/or aggressive has no place in the peaceful flock.