Why does Valencia's memory of Charlene haunt her each and every day in the novel, Valencia and Valentine?

Valencia and her friend Bethany bullied Charlene relentlessly, and after one particularly cruel exchange, Charlene jumped off a bridge and almost died. She became a paraplegic for a few years before dying of complications. Charlene’s memory has haunted Valencia every day since because she feels responsible for the girl’s death and undeserving of having a fulfilling life after ruining Charlene’s.