1) The (Magen Avraham in) Zayis Raanon (on Yalkut Shimoni) asks a similar question. It says ?? ??? ??? ?? ??? ???. What is the need for ??? ????? He says it fits according to Shmuel who holds ??? ??? ???? ??? ????? ????? ??? ?????? ?????? ????. Or, he answers, it’s a Mitzva (/Gzeiras HaKasuv -Minchas Chinuch 520).
2) The Shach (Sifsei Kohen, Parshas Shoftim) says even though the Rambam holds like Shmuel to a certain extent, he surely wouldn’t hold there would be killing in the times of Mashiach (even unintentionally, because it is implied in Makos 10:2 that a person unintentionally kills a person who killed intentionally, and when Mashiach comes there will be no people who kill intentionally, left to be killed, even unintentionally. And the Rambam says “?? ???? ??? ?????” and “?? ???? ??? ?? ????? ??? ???? ?? ? ????”. -Lubavitcher Rebbe.) See the Shach’s answer ?? ??? ???? that ???? is ????? ??????.
3) The Maskil Lidavid (on Rashi) states that (though in his opinion according to Shmuel the Pasuk is understandable,) according to the other Man Diamar there obviously needs to be an explanation for the need for ??? ???? in the times of Mashiach as he says ????? ??? ???? ????? ???? and it is stated ??? ???? ????. He offers that the ????? and promise of no killing and similar ?????? only applies to Yidden of those times, not to those who will become Geirim then.