Well that is a big assumption though we just don't know. Again, for all we have seen all this could be is dimension shifting, so he just hops to a different dimension, leaving his body behind a la Zero Escape.
If he is dimension shifting, then there are thousands of timelines out there where the ones he is supposed to save are the equivalent of Adolf Hitler. It's the thing with infinite variations of dimensions.
Come on, it's clear this is working as Steins Gate, Groundhog day, All you need is Kill, Madoka... whatever the scifi bullshit it uses, in all of them the previous timelines/dimensions are disregarded.
More importantly, I think I didn't connect to this series in the same way that I did, for example with Steins Gate, and it's because it doesn't get to make the illusion that all the dangers and bad things are for real, as since the start we had the time ability. In Steins.... well, it was only an illusion now that I have the hindsight, but as I was watching it, I had the illusion that things mattered, because Okabe was at risk of getting stuck on a timeline where he couldn't reach his goal, that it was impossible to save everyone and sacrifices had to be done along the way.
Though the system of 'checkpoints' are promising here in Re:Zero, I was thinking how it could be possible for the checkpoint to fuck Subaru over, if one activates without him noticing in the wrong time and place, making impossible for him to do something.
But in general terms, Subaru seems more a... tool in the plot, he has no agency, so it's hard to get angry at him. The way the system works, he can't explain his situation, he can't get help about it, he doesn't know shit of what's happening because he comes from another world, in comparison with mages and beasts in this world he is woefully underpowered, if he fails he has to try again and gain until he advances in the correct way, if he advances in the wrong direction you only need to kill him until he gives up to his 'fate'. He seems a tool of the Witch, with him being a key piece of her plan, whatever it is.