You ever saw little kid whom have a toy that they don't want, but don't want other people to have it either? Trust me, everybody have that possessive side to them. But, most reasonable guy, would not act upon such impulse, because...he have no right to act that way. If your ex crush act that way, because he is jealous, then it mean, he is a very emotional people, emotional plus possessive instinct mean...if you do date him, he would be a very possessive boy friend (I mean if he act that way when he don't even want you, imagine if he do want you.) my suggestion run, run away and never turn back.