I accidentally decapitated a few legionaries during a random battle in my bloodlust frenzy and now they are all upset. How do I get them to calm down?

Try getting arrested in the hold you're being attacked in.

Kill more of them until they bow down in submission and worship you as an angry god!!!

Jk, but yeah put away your weapon and hope they will be merciful by tossing you in jail.

Load an earlier save game and be more careful next time.

Get arrested.

If they don't give you an opportunity to talk/bribe them, the only solution is to reload to the last safe place i.e. just before you accidentally killed some of them. If your speech is higher than 90 you can bribe yourself out of most situations

Punch their noses let them no who's boss forgiveness is for the weak