If they were warned and know they were warned, you can't really place 'blame' on anyone but them. You can't say they weren't warned and it wasn't a failure in the warning system. There are unfortunate circumstances where people like the elderly can't get to a shelter.
I live in a house on a slab, meaning I don't have a basement, my neighbor does have a basement though and I can go over there assuming it isn't underwater at the time.
You also have to deal with 'warning fatigue'. When an area experiences a lot of tornado warnings and 'nothing happens' at their particular location, they may not take action when that one time something does happen at their location.