Lindsay Lohan?
I think you’re thinking of the saying, “if you can’t be a good example, you’ll just have to be a terrible warning.”
“warning to others” is the term being used but in this case the “to others” is implied.
Haha you beat me to it. I love demotivators and that’s one of the best.
Yours truly! 
Last week I finally found a legitimate reason to tell a friend “but we were on a break!”
Antithesis might work depending on the context. The latin roots seem to mean to set against - which fits the meaning.