Even if the beggar (odysseus) succeeded in bending the bow, what did antinous threaten?

He threatened to send him to King Echetus, the maimer of all men

He then continues to describe all the monstrosities that Echetus would do to him. Penelope however intervened and said that there is no need for such things since even if the old beggar managed to bend the new and polished bow, she would still not marry the beggar since she would be waiting for her husband to return.