If you set your ward to do lethal damage, and they kill a mortal, yes, they're self-defeating.
However, wards don't have to do lethal damage; in fact, they don't have to do damage at all. They reflect force; they can be set to just knock someone silly, with the extra shifts just going to the environment.
A wizard in a lonely farmhouse might well set a modified Veil ward; people just see an abandoned farmhouse, nothing worth noting, move along.
In a city, wards could just be a wall; no password, no entry. Or set to loop people through the Never-Never; try to enter a window, come out on the front porch.
Wards generally bar access through any point of entry. No loopholes. So, dig through the floor or chop through the ceiling...same results.