The Rambam (Rotzei'ach 7:8) writes that someone who killed accidentally and is presently protected by an ir miklat, doesn't leave even to save a life because as soon as he leaves the go'el ha'dam can kill him. The Ohr Samei'ach points out that the Rambam is coming to explain that since the go'el hadam can kill him, we can't impose on him to leave even if it is for pikuach nefesh. The Rambam seems to hold that a person isn't required to place himself in sakana to save someone else.
The Ohr Sameiach then continues to write that the Radvaz holds that a person is chayev to lose a limb in order to save the life of another jew. The Ohr Sameiach disagrees with the Radvaz and cites a brilliant proof from our gemara. The gemara says that witnesses who testify about a murderer and he is convicted based on their testimony, have no ability to retract. Rashi quotes an elaborate story with R. Shimon Ben Shetach who killed 80 witches, and to revenge their death, their relatives testified about his son that he is chayev misah. Although the witnesses eventually felt bad and regretted their false testimony, they had no way of retracting. The Ohr Sameiach questions, why weren't they obligated to amputate their arms, thereby being unable to fulfill the din of יד העדים תהיה בו בראשונה, based on Shmuel 45b who says that if the eidim lose their arm after the g'mar din prior to misah, the person is acquitted. Shmuel holds that if the eidim originally had arms and lost them, the convict is exempt since they must be able to carry out the punishment with the arms, as the Torah describes. The Ohr Sameiach proves from here that one isn't required to lose a limb to save the life of another, even if they are the cause of his death, and certainly if they are simply innocent bystanders.