Yeah kill moves can get repetitive, but if it is a special attack the player has a choice to use or not I think its better than one that is forced upon the player under certain conditions, which seems to be the norm for them. But you make an excellent point, giving the zombies more different animations is also time well spent.
Are you also going to be looking into the crawlers and their animations? Seems they often get stuck on objects or start to fly when they do their lifting up animation to get over objects, sometimes also resetting back to where they started because they could not get over the object.