The Mattapan neighborhood where a quadruple slaying happened in 2010 reacted with rage, grief, and disbelief to news that a Suffolk Superior Court jury was unable to reach a verdict on the murder charges against the alleged triggerman and acquitted the man accused of being the killer’s getaway driver. Boston police officers, ministers, and health workers walked up and down Woolson Street, where the murders occurred, hoping their presence would allay fears, calm nerves, and prevent violence.