Spoiler alert
Now Hitomi didn't actually appear much in the Madoka magica storyline only making appearances in a handful of episodes but almost every single event can be traced back to her.
Madoka meets Mami Tomoe therefore causing Mami Tomoe's death
Madoka meets kyubey in a storage tunnel and saves him from Homura in episode 1 of Madoka magica. If Hitomi had also been at the music store with Sayaka and Madoka she could've noticed their behavior and stop them from going in and while I'm sure Madoka and Mami would meet at some point it is implied in every other timeline that she fought Charlotte with no one else there and won the fight.
Sayaka Miki falls into despair and becomes a witch resulting in Kyoko Sakura's death
Why this event isn't completely hitomi's fault it is mostly her fault as even the knowledge that Sayaka was just a shell didn't completely through into despair it was the fact that Hitomi and kyosuke had mutual affection for one another. Sayaka Miki turns into Oktavia Von Seckendorf who Sakura tries to turn back into Sayaka however fails because it's not possible resulting in her death by self explosion.