I cant lie, the Grandma is kind of DUMB throughout the whole movie. She wanted to do this. "OH CAN WE GO OUT SIDE AND HAVE A CUPPA TEA". They collected acid rain and used it as water :face palm:. Then the old couple dies of radio-active sickness.
So I think that Hilda killed both of themselves.
Old people should understand the new generation.
Like when we will grow older we will adapt to what the youth does and learn how to survive a nuclear bomb or WW3.
That's my review guys. SHUSH