OK, let’s count all the different deceptions that occur in this story.
We're counting five pretty serious deceptions in this story. Each deception stems from the one before it. If the giant hadn’t disguised himself as a stone-mason, the gods never would have agreed to the deal, etc. But does that excuse the gods for breaking their oaths and killing the stone mason? The narrator of the story in the Prose Edda doesn’t seem to think so. He reminds us of these oaths three times. He even ends the story with a poem about how "broken were oaths then / bonds and swearing, / Pledges all sacred / which passed between them" (Prose Edda, Gylfaginning, Chapter 42, pp. 56). In Viking culture, promises were very important, and even a god, it seems, is supposed to keep his promises.