Who is ultimately responsible for tom's death & why?

The traditional answer to this question is Bob Ewell, who is abusive towards Mayella. Mayella's advances on Tom can be largely attributed to her own pain and insecurity, and Bob sees the encounter between them as an opportunity to take advantage of the racism that is already so prevalent in the town. Although Tom is eventually shot trying to escape from prison, Bob represents most directly the ideology which claimed Tom's life.

Mayella Ewells fault. If it wasn't for her curiosity, she'd never kiss Tom, and Bob Ewell wouldn't of accused Tom of rape.