These mathematicians (or very mathematical non-mathematicians) died young (<50) and/or from non-natural causes; I did not see these names:
Yom Tov Lipman Lipkin (c.1846-c.1876), smallpox. Codiscoverer of the Peaucellier-Lipkin linkage. A son of the famed Salanter rabbi, founder of the Mussar movement.
Hermann Minkowski (1864-1909), appendicitis. Number theorist, but best known for his work in relativity.
Janina Hosiasson (1899-1942), murdered by Gestapo. Foundations of probability and inference. Wife of:
Adolf Lindenbaum (1904-1941), murdered by Gestapo. Mathematical logic, published little, but collaborated widely, especially with Tarski.
Duro Kurepa (1907-1993), robbed and murdered during the post-Yugoslavia wars.
Carol Karp (1926-1972), cancer. Major founder of infinitary logic.
John Nash (1928-2015), car crash, wife also killed. Career cut short by schizophrenia.
Karel de Leeuw (1930-1978), murdered in his Stanford office by his perpetual grad student. Analyst.
Richard Montague (1930-1971), robbed and murdered at home, apparently by strangers he picked up cruising. Logic, philosophy, linguistics.
Robert Hamilton Boyer (1932-1966), murdered, a Texas tower massacre victim. General relativity, Boyer-Lindquist coordinates.
Frank Adams (1930-1989), car crash. Major topologist.
Alan Mekler (1947-1992), cancer. Applications of set theory and model theory to algebra.
Boris Weisfeiler (1941-?1985), disappeared while hiking in Chile during the Pinochet regime. Assumed kidnapped, tortured and murdered.
Zoltan Balogh (1953-2002), sudden illness (with history, also father Tibor died young). Set-theoretic topologist.
Elizabeth Gardner (1957-1988), cancer. Theoretical physicist specializing in disordered systems, her most mathematical work was the statistical physics of neural networks.
Moez Alimohamed (1967-1994), robbed and murdered outside his apartment. A UPenn math grad student, he had just finished his first paper/thesis. Published and degreed posthumously.
Michael B. Cohen (1992-2017), sudden illness. Up and coming superstar in CS, especially algorithms.