The Flash has established the existence of different timelines, but Savitar’s was the worst one that Barry Allen created. Time travel was introduced all the way back in season 1 of The Flash, with the scarlet speedster accidentally running fast enough to travel to the day before. However, Savitar’s creation is what ultimately led to the darkest and most devastating timeline to date on the show.
Barry quickly realizes the repercussions of time travel, though that hasn’t prevented him from time traveling every so often — be it to stop Reverse-Flash from killing his mother in the past or to confront the very same villain for mentoring his daughter in the future. These time-travel adventures have created alternate timelines and changes (both big and small) to people’s lives and events. For example, after running back to save his mother, Barry created “Flashpoint,” a timeline where his mother was still alive, his father didn’t go to prison, Cisco Ramon ran his own business, and Wally West had taken up the mantle of The Flash.
Without realizing how the events of “Flashpoint” would reverberate through to the present, Barry fixed his mistake before running back to the point in time that he left, only to realize that his actions had created problems that affected other shows in the Arrowverse. What’s more, Nora West-Allen saving Barry in the present accidentally moved up his disappearance during "Crisis on Infinite Earths" from 2024 to 2019, so the ability to mess with time and alter events is something that clearly runs in the family.
In the case of Savitar, the villain is a time remnant of Barry’s, created in the future to help defeat the very same villain he became. It’s complicated, but Savitar plotted to kill Iris West in season 3 to cause the real Barry Allen pain and suffering after being rejected and unloved for so long. In a future without Iris, Barry was despondent, heartbroken, and miserable. He stopped being The Flash, abandoned his job, and left his family and friends behind. His failure to defeat Savitar and save Iris left his life in complete disarray. How could he go on knowing he hadn’t been fast enough to save the love of his life?
The rest of his friends and family weren’t exactly faring any better in this timeline, either. Iris had been dead for seven years; Cisco had lost his hands fighting Caitlin, who had transformed into the evil Killer Frost, and could no longer use his powers as Vibe; Wally was practically catatonic after trying to fight Savitar on his own. The only person who was thriving was HR Wells, who was profiting off of his romance novels and was now the owner of the coffee shop Jitters. When comparing it to every other change or alternate timeline on the show, the future where Savitar successfully kills Iris is harrowing and fraught with misfortune. Plus, having Iris die at all at the hands of a villain created by Barry was not a good way to go either. The Flash has seen better days and the scarlet speedster will hopefully never create another future as bad as this one again.