It's not worth it to a coyote to take on a full grown deer. However, if they can't find any other food, they will. It's more likely that they will take out the weakest or youngest of a herd when they try, but I have heard other hunters claim that they watched a pack of about a dozen tear down a 12 pt buck that looked healthy as a horse. They shot 4 of them before they went and claimed the rack. As stated before, they won't do it if they don't have to, but if properly motivated, you'd be surprised what they can kill.