Évariste Galois, the pioneer of Galois theory, died at the age of twenty from wounds sustained in a duel; the duel was apparently over a woman Galois was interested in.
Oswald Teichmüller, a brilliant mathematician who made contributions to complex analysis and invented the theory of Teichmuller spaces, was in fact a fanatical Nazi who died fighting for Hitler on the Eastern Front in $1943$.
What are some other examples of mathematicians whose lives ended in bizarre ways?
... said Archimedes before he was killed by a roman soldier with his sword, back in 212 BC during the capture of the city of Syracuse.
*) "Don't disturb my circles" (Latin)