So you asked someone to do something*, and though they agreed beforehand, they did not follow through. You confront them and ask them why they flaked out. Given that they have no good reason and you'll be annoyed at best either way, would you be more upset if they say they just didn't care from the start or if they said they flaked out specifically to upset you, because they felt you'd slighted them in some way the day before?
(*Said thing is not life/career-threatening or particularly emotionally charged.)
Alternate question: favourite mythological creature you know of?
--I think I'd be annoyed but willing to ask them why they were upset w/ me, and if I had a "wow, I was totally a jerk" moment then I'd try to reconcile with them. If not I would probably facepalm and try to talk it out anyway. If the person was my friend, at least; if it was a near-stranger I would probably just ditch them.
--Thestrals. Harry Potter-derived wizarding world mythology is a mythology, damn it.