Well on second thought.....
This was a temp job after a layoff and the new job is back where I was before. (temp, but still over a year).
You do not owe any employer anything other than an honest day's work for an honest day's pay. If you did that while you were there, then you are even....
However...... Let's just say you were furloughed from your last job at a 121 airline, and you told the chief pilot of your current corporate gig "I've had enough of the lousy pay, sitting reserve, commuting, no upgrade, being furlouged, etc...) If he hired you on good faith that you wanted to start a new career as a corporate pilot and then sent you to get typed and the you left right after your first recurrent.. Well, me personally, I would be pissed at myself for having fallen for such a common lie. There are thousands of pilots who found Jesus (ie. decided that being a corporate pilot would be a better lifestyle) after they got furloughed, then decided to slip back into bed with the Devil when she <-(EEOC) came a callin' again.
Now, don't get me wrong. I don't know your situation, as you have been vague so far about whether or not they sent you to initial and how long you were with them. But... your situation sounds awfully familiar, and it is the reason why many smaller flight departments will not touch a guy with the "121 stink" on them. There are usually only two scenarios that would drop a 121 guy on the doorstep of a corporate operator. 1. I was furloughed. 2. I was fired (wow - musta been pretty bad if your union couldn't get you out of that one) . The third and less common is "My airline shut down." That may be the guy that some will take a chance on.
I am not passing judgement on a 121 pilot's flying ability or anything else here. They are usually very good pilots and easy to get along with. The reason that some small outfits will not touch them is that it is a big risk (in terms of them falling back into the arms of their former employer). Depending on the aircraft, a type rating can be a pretty pricey investment.
In the end, it's all business. You do not owe them anything other than two weeks notice. If you intended all along to return to your former employer, and were less than candid with the corporate operator when he hired you, then perhaps you suffer from an integrity problem and you are looking for solace here. Again, I do not know your situation. The fact that this is bothering you enough to post for advice here, tells me maybe you do have some integrity. Sometimes a Captain has to make decision, neither one of which is ideal. Good luck.