Ben having nightmares about Malware in the past have taken a bit of a toll on him when Guardians starts, likely remembering Feedback.
The flashback for Feedback's initial destruction was devastating for everyone involved; Ben was traumatised, Gwen and Max could only hold him to comfort him and Azmuth could only watch from afar after Malware's supposed demise.
Ben and Skurd later find a recording of when Azmuth removes the Omnitrix from his 11 year old self.
Billy's backstory is treated a lot more tragically, especially with his parent's deaths at the hands of Computron.
Rook's family lost a member because of a rogue plumber; Da's brother, Rook Fra. A bit of a Heartwarming moment occurs when Ben makes a statue.
Carl Tennyson getting injured by Captain Nemesis with Ben watching in horror.
It's revealed in Ben 23's earth that Grandpa Max died when he was rather young.
