I said yes to an offer last Thursday, but just got a much better opportunity yesterday. I haven't started or done any onboarding yet. Part of me feels like I'd be a horrible person to bail, but the other offer is just so much better. Has anyone backed out of a job after accepting?

Be the horrible person. Companies are horrible to employees all the time and they don't give it a second thought. There is nothing at all wrong with making the right decision for you. Take the offer before it gets rescinded. Congratulations on your new job!

I left two weeks after starting when I was in your position. Backing out doesn't make you a horrible person. It means you're prioritizing yourself, as you should be. That company wouldn't bat an eye about firing you. You shouldn't turn down more money and a better opportunity just to make things easier for them.

Prioritize yourself!! If it’s better and you haven’t even done onboarding yet I’d say pump those arms and run as fast as you can to that better opportunity

Take the better offer. You owe no loyalty to a company you haven’t even logged into yet. It’s business, not a blood pact. Just send a polite email and move on.

Do whats best for you, companies are not loyal to anyone. Don't feel bad either.

You would likely be miserable and resentful if you stayed on out of some perceived sense of obligation. They could rightfully terminate your position a week or two in, don't sweat it.

Do what you think is best for you today! Go for it !

This is your life. You need to make yourself a priority and decide what is best for you. Do follow your heart.

I went through this. Had an offer, then had a different offer with the same pay level but at a company that did not tell me through *four* rounds of interviews how dysfunctional the dept I'd accepted a job with was. I totally burned a bridge by renegging on my acceptance of the original (dysfunctional team) offer. It came back to bite me HARD when the less-known, not-a-household-name company I decided to go with laid me off after a few months (me and many others) because I was the most expensive on the team. Now after two months I'm still unemployed because the market stinks. I suppose my advice is, go with your gut here. Make as much $ as you can in a good working environment. You are potentially screwed (or not screwed, depending...) either way. It's Magic8-Ball stuff, this market. Ugh.

Another thought: be as certain as you can that the new offer is the better position. You also don't want to burn a bridge only to find out after a short time it wasn't exact as was represented, and in a bad way.

I have a different point of view, it is about honouring your commitments.
If you committed and signed a contract you should honour your commitments.
I advise you go & start the job you accepted and signed a contract, the other new one also they might fire you after couple of weeks as people are advising you to be opportunistic and just go with few extra K of $.
I am not giving this opinion just for the sake of being horrible, market is already horrible and both employers won't give it another chance to fire you if they see so, however honouring commitments is something you need for yourself and maybe you would inform them them later that you dropped a better offer to keep your commitments, I trust this will leave a very good impression and they will consider you as an asset as in the today's market many employers started to invest in quality employees more than the achievers with attitude or woth no values.

It is not about being emotional, but I am telling you from a managerial level we value and seek high valued employees more than performers with attitude, a low performer can be trained but a low quality person is hard to fix.