The mishna says that a person who killed b'shogeg cannot leave the city of refuge even to testify for a mitzvah purpose, "and not even to testify for a capital case, and even if the Jewish people need him, and even a general of the army like Yo'av Ben Tzeruya cannot ever leave". Many commentaries ask that this seems to contradict a famous principal that nothing stands in the way of pikuach nefesh. Why would he not be able to leave if there is someone's life at stake, and he might be able to save him?
The Cheshek Shlomo (Rav Shlomo Vilna) interprets the mishna to be saying something very different. He assumes that surely one would leave the ir miklat to save the life of another Jew, but he would be required to return. The mishna is only addressing if the exemption to leave temporarily would be a permanent exemption, to which the mishna says - אינו יוצא משם לעולם, meaning, he cannot leave permanently (rather than meaning that he cannot leave ever).
The Ohr Sameiach takes a different approach. Although by leaving he may be saving the life of one or many Jews, he would also be risking his own life. One is not required (and according to this approach perhaps not even allowed) to risk their life to save the life of another. Therefore, we can't impose on the one who killed b'shogeg to leave the ir miklat to save the life of another Jew or even the entire Jewish people, since by doing so he would inherently be risking his own life. R. Meir Simcha is medayek this from the language of Rambam
ואפילו כל ישראל צריכין לתשועתו כיואב בן צרויה אינו יוצא משם לעולם עד מות הכה"ג ואם יצא התיר עצמו למיתה
The Rambam makes a point of saying that by leaving, even for the best of reasons such as saving the life of a Jew, he is being matir himself l'misa - causing his own death. Since he isn't obligated to risk his own life, he isn't obligated to leave.
We generally assume (based on a beautiful teshuva of R. Moshe in Igros Moshe) that one is not obligated to risk their life to save another, but has the prerogative to choose to do so (and it would be considered a big mitzvah). According to this assumption, the Mishna and Rambam would only be saying that he wouldn't be obligated to leave, but couldn't be saying that he isn't allowed to leave. The problem is that the language of both the mishna and Rambam imply that it is forbidden to leave, even for pikuach nefesh purposes.
See Aruch HaShulchan 425:57 who raises this very question. He understands that Rambam like the Ohr Sameiach and therefore says it should be permitted for him to leave for pikuach nefesh purposes, just that he isn't obligated to do so.