离开 = leave (perhaps a chance to return)
离別 = to separate, to say goodbye, to depart. (perhaps, never to return)
离开我 = leave me! You cannot say "depart from me" to have the same meaning.
离开人世 = to leave the world i.e. to die but perhaps can return as a reincarnated soul. 离別人世 same meaning but most probably will not be able to back in any form etc.