I'm an infamously tough grader, too.
I once gave a young lady a D on a college term paper and wrote, "This is a gift."
She dropped out of college.
;-)
LOL.
My first paper as a freshman in philosophy 101 (it was ultimately my major), I got a C on which the prof wrote, "I wouldn't be surprised if you thought you worked hard on this, students tend not to know what real work is."
I was seriously offended for 10 minutes, until I thought about how right he was. My 25th reunion is next year. Some things stick with you.
That prof turned out to be my advisor when I went for the honors degree, (I passed), but I did get to spike his a$$ during a student / faculty volleyball game.