Nothing is known about Beckett before the apocalypse.
While Beckett and Isabelle were on a resupply mission, they witness the aftermath of a power plant meltdown. It killed the people inside it making them turn into radioactive walkers. Beckett panics about turning into one of them. Isabelle then shoots Beckett because of his panicking. However, she failed to put Beckett down and as a result, when Beckett reanimated, he joined a herd.