Osberht accepted Ragnar's offer because he knew that the Vikings would be stupid to kill him, they would lose a lot more doing that as this would only guarantee their destruction.
Osberht was allowed to enter with his translator so they could converse a lot easier.
Everyone glared at Osberht, they wanted his blood but Osberht wasn't fazed by this.
He walked like a man filled with confidence and Ragnar could tell that this man was a true King.
And he too could tell that Ragnar was one with a single look in his direction.
However, he caught a glimpse of his brother and was surprised that they hadn't killed him.
The rule of conquest was killing the King and the entire Kingdom fell but Osberht realized that these men weren't mere barbarians, they didn't act on impulse.
He didn't even know if his brother was glad to see him but the look in his eyes said otherwise.
Osberht was banished from this very land he was about to save, his home.