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.