3. It doesn't matter if he has multiple curse marks and can come back. He can't physically do it himself and someone has to unseal him. If he is killed and is just kinda waiting inside a curse mark, it still removes the immediate danger that he poses and hell, if nobody revives him ever then he STAYS inactive and is essentially dead. All Itachi would have to do at that point is hunt down anybody that has his curse marks (lol, since Kimmimaro, Anko and Part I/early Shippuden Sasuke were the only ones to have it that were actually somewhat strong, that's laughable) and kill them and essentially removes the threat of Orochimaru forever.
