The Umbrella Academy revealed in season 3 that Klaus has the power of immortality, which makes Ben's death even more heartbreaking. Although Ben's cause of death is still unknown, Klaus' inability to die adds a layer to Ben's character. Ben's death is referred to in season 3 as The Jennifer Incident, which was previously only known to be a mission gone wrong. With the new knowledge of Jennifer and Ben's connection with her as well as Klaus' power of immortality, viewers can piece together why Ben's death took such a toll on the family.
Klaus has always been connected to the afterlife, as viewers saw throughout season 1 and 2 of The Umbrella Academy. Klaus' ghost powers allowed him to communicate with and summon spirits into the real world, both voluntarily and not. These abilities hinted at his liminal state, but until season 3, Klaus believed his survival of uncanny circumstances to be purely luck.
Ben's attachment to Klaus seemed to be because he was the only one Ben could talk to, as the other siblings cannot see Ben. However, knowing that Klaus was immortal during the Jennifer Incident could give even more meaning to their connection and why Ben stuck around for so long after his death.
Klaus Being Immortal May Make Ben's Death More Tragic
Since viewers still don't know what happened during the Jennifer Incident, one theory suggests that Ben's death was a sacrifice to save her. The sketches that Viktor finds in his room in season 3 show that Jennifer and Ben knew each other in both timelines, which suggests that the original Ben may have sacrificed himself to save her. Ben dying to keep someone else safe, whether that was his siblings or Jennifer, may not have been the only option. If Klaus sacrificed himself instead of Ben, no one would have needed to die, and Klaus most likely would have returned unscathed. Klaus learning of his immortality then makes him partially responsible for Ben's death.
Klaus Knowing He Could've Saved Ben Makes Their Bond More Meaningful
While there are many theories for how The Umbrella Academy's Ben died, Klaus' immortality impacts his relationship with Ben regardless. Ben's ghost has been attached to Klaus for years by the time Klaus discovers he is immortal. By making the realization that Klaus is immortal, both characters have to face the reality that Ben might not have had to die and Klaus' view of Ben likely shifts and softens. In addition, Klaus is continuously working through his relationship with the afterlife and the void he enters every time he dies, which he gets better at finding his way out of each time. This shows that he has experiences that only Ben can understand as they are the only two who have experienced death.
Additionally, Klaus doesn't tell anyone he sees Ben and keeps Ben mostly to himself throughout the series, so they also understand each other more closely than the other siblings. Though the two have tension throughout the series, they are the only allies for each other in many ways. By recognizing that he could have saved Ben if he knew about his powers, as well as their shared experiences in the afterlife, Klaus and Ben are bonded in ways the others aren't.
Klaus' new powers clear up many mysteries about his earlier storylines in The Umbrella Academy, as well as how he survived so many bizarre circumstances. Even more though, Klaus' immortality deepens the audience's understanding of Ben's death without revealing what exactly happened. In addition, Ben's death seemed to completely shift the family dynamic for the Hargreeves siblings and the knowledge of Klaus' role in the Jennifer Incident complicates that even further.