Some goats are extremely malicious.

Or the goat is contained in too small of an area for too long. Much like an inexperienced person without supervision...

> is not just incompetent, but where he will actually cause damage - for instance because he has ulterior motives.

That made me think it was more. And anyway, as much as the fox does with chickens, the goat knows it will set about to kill and eat plants. There's no real reason to call the fox knowingly malicious and the goat not.

In Finland we have almost exactly the same: Goat guarding the gabbage patch.

And to the right, we say "Let the wolf guard the sheep" :)

The German proverb is also about a "Bock", same word, same meaning.