I think its because <PERSON> wanted a long time to be with <PERSON> for his Resurrection, and they needed time to set up for his resurrection. I believe he intended to use the portkey to come back to Hogwarts disguised as <PERSON> (using a polyjuice potion), and kill <PERSON> when he wasn't expecting it. My evidence: First: I want you to think. Think about all of the portkeys ever used in Harry Potter. The only one that was two-way (it will take you back to where you left from as well as where you are going) was the Triwizard cup, which supports the part of my theory about <PERSON> getting into Hogwarts. Next: Why would <PERSON>, newly resurrected after 15 years of being "less than the meanest ghost", risk fighting <PERSON> in a duel, and give him back his wand for it. 773
He doesn't have to prove that he is better than <PERSON>, since the Death Eaters fear him and wont think anything of it if he doesn't battle <PERSON> (well, out loud and with actions, at least), so he could have kept <PERSON> tied to the tombstone and killed him, witch would have the same outcome as the duel (what <PERSON> wanted to happen). Instead, he lets <PERSON> down and gives him his wand so that they can duel. I believe he did this so that he could win <PERSON>'s wand, so that when he went over to Hogwarts, he would have the full, working disguise. <PERSON> never really understood wand lore, so he might have heard that a wand will work for you if you win it from its owner, but being <PERSON>, he probably thought that that meant he has to battle the person, and kill them to win. That isn't the only reason why he would wait till the end of the year, and use the triwizard cup as the portkey. If <PERSON> suddenly went missing, <PERSON> would be able to guess it was <PERSON> who caused it, and deduce that <PERSON> is an impostor, all of which, as <PERSON> states, is the last thing <PERSON> would want once he was resurrected. 773