Or he is making a threat that could influence the outcome in his favor. Making a threat is not breaking the deal.
And who says Trump wanted a level playing field?
Trump did, actually:
Asked again if he was open to an independent run, he repeated, "Well, I'm going to have to see what happens. I will see what happens. I have to be treated fairly. You know, when I did this, I said I have to be treated fairly. If I'm treated fairly, I'm fine. All I want to do is a level playing field."
Although in this Fox news story covering the signing, I saw no mention of him talking about having to be treated fairly "when I did this." And the pledge he signed reads:
"I, ________, affirm that if I do not win the 2016 Republican nomination for President of the United States I will endorse the 2016 Republican presidential nominee regardless of who it is. I further pledge that I will not seek to run as an independent or write-in candidate nor will I seek or accept the nomination for president of any other party."
And according to this Breitbart story, "Trump said he would not rip up the pledge at a later date."
The pledge was a sham, it seems to me -- both the GOP that put it forth and any who agreed to sign it, participating in a sham publicity stunt.