The events of Lost in Space season 3 led to a climactic showdown between SAR (Second Alien Robot) and the Robinson family, but it was actually Robot who saved the day. Over the course of Netflix's sci-fi show's three-season story, the Robinsons became wrapped up in a plot with alien robots. It all started due to the robots secretly enabling humanity to travel across the galaxy after crashing on Earth. These robots are led by SAR and spent much of Lost in Space fighting humans, with SAR having a special interest in finding Will Robinson due to his connection to Robot.
SAR's return in Lost in Space season 3 came as somewhat of a surprise based on what happened before. Lost in Space season 2 saw Ben Adler presumably kill the robot leader after it impersonated Robot. Ben and Maureen Robinson left SAR's body behind so no one else could use it, but Lost in Space season 3 revealed the robot minions found the remains. Once SAR was back to life, the robot went all-out in its pursuit of Will. This led to the reveal that SAR was responsible for killing the being who created the robots. SAR also wanted to kill Will because he proved he could free robots from their programming, which made Will their leader in the robot's eyes.
As Lost in Space season 3 came to a close, SAR's legitimacy as a villain was solidified. The robot appeared to kill Will Robinson and then successfully invaded Alpha Centauri. It was during this attack that SAR and Will faced off once more, but this time Robot wasn't around. However, the Robinsons' friendly robot companion still wound up defeating SAR. The evil robot attempted to kill Will again by doing exactly what it did before: stabbing him in the heart. What SAR didn't know was that Robot infused part of himself with Will's heart to save his life before. Once SAR made contact with Will's artificial heart, Robot was able to upload itself to SAR's body and take control.
Since Lost in Space season 3 breezes past how Robot's technology became part of Will's heart and the exact makeup of the alien robot's origins, exactly how this happened isn't specified. This is likely the result of the show only getting three seasons to tell its story, as the origins of robots might have been fully explored if there were more time. Still, Lost in Space season 3 didn't briefly set up how Robot defeated SAR. The series showed the robots taking a piece of SAR's original wiring and placing it inside a new body. It seems that there is core data in each of the robots that make them unique, so Robot must have found a way to upload itself to SAR's body like a virus.
Although Lost in Space season 3's ending indicates that SAR is gone once and for all, it is up for debate if Robot really did defeat the evil robot. SAR should be truly dead if Robot's code completely erased all presences of SAR in that body. However, it is quite possible that Robot simply locked SAR out. That would mean that a piece of Lost in Space's villain still lives inside Robot's new body and could return at any time. Since the show is over, viewers will never learn if that were to happen, which allows Robot to end the series as the true hero.