San Francisco 49ers return man Kyle Williams might be the most hated man in the Bay Area today -- and now his family is stepping up to defend him after a barrage of nasty threats from angry Niners fans. 

Williams' two monumental turnovers contributed heavily to yesterday's loss to the Giants -- and his stepmother, Jessica, tells TMZ no one feels worse about the game than the family ... saying they are "devastated." 

For those who don't know, Kyle's family is a big deal in sports -- his dad is Kenny Williams ... GM for the Chicago White Sox.

Jessica also referenced the wave of angry tweets aimed at her son, saying ... "We also know there are some people who have singled Kyle out, but there are many, many more that support him. We will rally around Kyle and keep him strong!"

Jessica added, "Kyle is a great athlete and we're looking forward to an outstanding season next year!"  If you missed it, here's how this season ended -- Niners fans ... look away.