I don't remember where I read this, probably in a printed anthology of science fiction from the 1970s or 1980s.
A woman is trained to be a handler for a powerful psychic with limited intelligence. He has lots of abilities and raw strength, but I seem to recall this man could be dangerous if not controlled rigorously. I'm a bit fuzzy on how she "handles" her charge - through drugs or intimacy or just because he was manipulated to trust her implicitly.
For some reason, she is no longer able to control him properly (was she dying?) so at the end of the story she holds him close and tearfully cuts his throat to make sure he is not a threat to anyone.