Backing out after accepting an offer ("reneging on the offer") is unfortunate but happens more frequently than you think. Imagine that you accepted a job offer, but two weeks later, you finally get an offer from another company that is better in every way. What do you do?
First, prevention is the best cure: try to get all the offers around the same time.
If the timing mismatch is unavoidable, you should do what’s best for you: don’t worry too much about the company.
In the US, employment is at-will, so there’s nothing legally preventing you from joining. Of course, you may have to give back any kind of signing bonus.
There may be reputational damage: talk about why the opportunity is incredibly unique and you hope to work with them again. Reneging on a large company is not as big a deal compared to a startup.