If Melissa sees killing Migen as killing Nine, she won't be able to do it. Or maybe he wanted to create the situation of (I'm the real Nine, kill him.. No!! I'm the real Nine) But he didn't think Nine would change his outfit. Migen needs to die, he won't allow Nine to go out of this situation alive..
I am Very Concerned about why Migen would decide to change his clothes to look like Nine before coming to meet everybody. Talk about sus.