The Mass Effect games have seen a lot of deaths, and many of them depend on the choices you make as Commander Shepard. Mordin Solus, the affable salarian scientist, is one of the hardest to recover from.
Salarians are an intelligent, amphibious race in the Mass Effect universe with slender, elongated bodies, large eyes and, two horns. As bizarre as it may sound, their mannerisms and speech are rather cute. At one point in their history, they made the (questionable) decision of creating a biological weapon called the genophage that could have potentially caused the extinction of another sentient race, the krogans.
In Mass Effect 3, assuming he’s survived to that point, Mordin is working to cure the genophage so that a truce might be reached between two warring races. Not everyone wants the genophage cured, and Shepard can choose to shoot him to keep him from doing so. If Shepard allows him to forge ahead, Mordin still dies in an explosion as part of a sacrificial act.