Everyone says "Stormcloaks bcuz Imperials tried to kill me." This is an objectively correct answer,
HOWEVER
>has anyone considered that maybe you were a mass-murdering piece of shit (or similar) and deserved to be beheaded, and the Helgen incident with Halvor gave you a second chance to (maybe) seek redemption?
That's a possible justification for going Imperial. No one ever mentions it.