Archimedes

Archimedes had great importance in mathematics for his discovery of formulae for the areas and volumes of spheres, cylinders, parabolas, and other plane and solid figures. He founded the science of hydrostatics, but his astronomical work is lost. He was killed at the siege of Syracuse by a Roman soldier whose challenge he ignored while immersed in a mathematical problem.

Well, isn't that the ultimate irony? The same thing he dedicated his life to killed him.