Question: I applied for Company A for a job. I applied also for Company B.

Company A has given me an offer. I accepted it, and I canceled all of my other applications, except to Company B. I did it because Company B was a much better job, but I've seen only a small chance for a success.

Company A sent me a contract to sign, and started to organize the starting procedure.

Meanwhile, also Company B has given me an offer. It was a surprise, but I had luck.

Now I need to cancel the procedure for Company A.

Yes, on the paper and on the law is everything okay. I got an offer, I am free to reject it.

But, the sad truth is that I was unthinkably dishonest. They honored me with a job, they trusted me, and I slapped them on the face. That is the truth.

I am ashamed.

What the heck could I do? What?

What should I say to them? I think, I will be forever blacklisted at Company A, and I deserve it, but what should I say them?

Answer: 
> 
> But, the sad truth is that I was unthinkably dishonest. They honored me with a job, they trusted me, and I slapped them on the face. That is the truth.
> 
> 
> I am ashamed.
> 
> 
> 

No need to be ashamed, thats just business. You are looking out for your best interest, the company is looking out for theirs..

Since you haven't signed, there's no legal obligation on your side.

> 
> What should I say to them?
> 
> 
> 

Thank them for their interest in you and their time spent on the application process and tell them you found another job that's more suitable to you.
You could add that they might keep your record for future opportunities.

That's all there is to it - no need to loose sleep or get grey hairs over it, your move is not so uncommon and completely understandable..
If they choose to blacklist you, it's very unlikely that whatever you're thinking of saying is going to change their mind.

That said, if you're ready to face the consequences (burning the bridges, to say the least), go ahead and inform them about the change of mind as soon as possible. That's the most you can do - to minimize the impact of your rejection and allow them sufficient time (compared to a delayed intimation) to find a suitable replacement for you.

Do not go into explaining anything - it's likely that it's going to make things worse - thank them for the opportunity, state your choice apologize and move on.