Say you have one goal. For example, to run a mile below 4 minutes. You train hard, do all your best to accomplish that goal, etc.
Somehow there is a person who does not want you to be successful: they keep telling you that you are uncapable of running that fast. Moreover, they do all that they can to sabotage your efforts: hide your shoes, etc.
The day of the big race comes and the sabotage works: you finally cannot make it because the night before that person kept ringing you on the telephone so you couldn't rest properly.
Then, that person comes and says: See? I told you that you wouldn't manage to make it!
Is there a sustantive, adjective or idiom to describe this? I was thinking about cynical but it kind of lacks the sabotaging part.