I'm catching up and right now watching the crossover and its so weird hearing the legends lecture Barry about changing the timeline. How many people have they killed? How does saving some ones life>>>> killing multiple people
The Legends don't radically change the timeline for selfish reasons/are aware of changes they make. Barry literally only knows he's changed things afterwards when he talks to people, and even then has no clue what precisely needs changing.
For example -
If Barry went to 1942 he wouldn't know he had to kidnap Einstein in the first place, and even if he did he'd never realise that he had to reunite Einstein and his Ms.
Now if Barry had a computer...
But the real truth is that Barry was meant to be the time related part of the Arrowverse on his own, and now with Legends they've changed that.