I remember seeing a promo for what seemed to be a made-for-TV movie from the mid ‘70s. It was probably CBS or NBC. The plot involved an older couple who were caring for a grandchild or adopted child.
One of them (probably the man) was secretly poisoning the child, and the other one began to suspect. There was a particularly shocking shot of a woman, in an apartment at night, with her hair on fire. She was screaming, not trying to put it out, just screaming and burning. I remember scrambling to the T.V. to shut it off immediately. I was haunted for months. I never knew what movie or show it was.