Creepy behaviour is behaviour that tends to make others feel unsafe or uncomfortable.

It would be really good to have a definition that had some shreds of objectivity to it. As it stands your definition simply assigns to one person the responsibility for another person's feelings.

Um, no. There is legal precedent for this phrasing in related contexts, albeit with the understandable proviso that the behavior must be "reasonably believed" to be threatening. This is pretty much what we're dealing with here: the whole problem with creepy behavior (as opposed to merely being awkward or anti-social) is that it puts people's personal safety at risk.