A thief who walked into an experiment by accident and came out able to become sand, who is doing all of it for a sick daughter.
Sandman is the most sympathetic villain the Raimi films made, and the least interested in being one. Flint Marko is stealing to pay for his daughter’s treatment, and the transformation happens to him rather than being chosen. The sequence where he first pulls himself back into a human shape is the best piece of effects work in the trilogy and has no dialogue at all.
Spider-Man 3 then makes him responsible for Ben Parker’s death, which is a retcon the film needed and a great many people did not want.