This "warning them beforehand" fig leaf only works if you think of everyone as fit healthy and mobile.
Anyone with disabled people, chronically ill people, terminally ill people and elderly people in their own lives knows it's not that simple.
Most of us don't have people physically weakened by famine in our own lives but it doesn't take Einstein to know this is a problem too. And from NGOs we know there's a lot of parentless children and a disproportionate number of child amputees in the mix as well.
If your response to this many civillians being killed is "it's their fault for not getting away" you need to examine your logic, I think.