Physical revenge wouldn't be enough for the twins. What they did would be much worse for Hikaru, knowing that the person you loved so much that you caused their death when they broke your heart...actually loved you. He ended Ai's life without even knowing the truth. And now that he does, he wants to take it back so badly but he can't.
Because people die when they are killed (unless they pull an Ai reincarnation or something lol).