He will kill and not pass over.

If a Gentile or an Israelite comes and threatens a man's wife or she marries him against his will or he kills Reuben, according to the Maimonides, there is no exemption and the permission of eternal life. Should Midian kill and not pass over, she should not listen to his will to rape her even though Reuben will die because of this and he will also kill and not pass over. Or perhaps not, and she has no permission to save herself from incest by having someone else kill Reuben.
And not blood money 253 in the matter of murder. If a person commands Reuben to kill Shimon, and if not, then he will kill Levi. Here it seems that it is better not to kill Shimon, because in doing so he will also violate the prohibition of murder and one soul will also be lost from the world. But if he does not agree to kill Shimon, then only the soul of Levi will die and he will not be prohibited from murder. And this is not the case in the picture of incest, where on the scale there is only incest on the one hand and on the other hand the loss of a soul that was not his fault.