I am a recent graduate, and I had a job offer a few months ago, which I accepted readily. I do not have any binding contract with the firm though. Now, this other company is interested in me and, I would say, at the verge of making a better offer to me. But my work at the first company starts in about a week's time.

Is it bad on my part to be looking for "better opportunities" once I have "committed (non-binding though) to this company? How would the company take it once I tell them of my decision to leave the place on the first day at work? I know these employers wouldn't probably think this hard before firing anyone, but I just can't not think about it.

I appreciate your responses.

depends really on you. if i were you, i would go for the first one. once committed, always committed.

just imagine how would you feel if you were to told in your first day at your office that they've found a better candidate in your place :D :P

Anything verbal is a committment in USA.

tell your first company about the other offer and you might just get a better offer from the first company!

However, it's always great to stick to where you have committed yourself. You don't want to form any bad relationships starting out. The business world is not that big, and you don't know in what form a stupid mistake like switching companies at the last minute can come back to haunt you later in your career. Connections are important, and any bad decisions can ruin your future prospects. Even the second company might have doubts about you; you are not reliable if you ditched another company like that!

The thing is I did my internship with the second company last summer. They know me there already.
Honestly speaking, I accepted the first offer as a safety net. But after I accepted the offer, I did not even bother to look for other jobs. But now this place, where I did my summer internship, asks me if I want to come back and wants to give me better offer.

The second company knows that I have the offer from the first company.