A speeding train, a blinding rainstorm, and the twisted remains of a car would haunt his life.
His petty decision had cost the life of his prom date. That event was such a deep cut that he spent the next twenty years living like a hermit, working on a speculative theory to manipulate the relationship between matter and energy, and thus alter the reality of time. When his efforts paid off, and the technology to jump time was in his hands, it had seemed easy enough to jump back and fix that one fatal mistake.
But nothing is ever that simple.