i think most people are wrong as to the reasons why kallen is upset.

She's mainly upset that he deceived everyone that Euphie was a mass murderer when in fact she was Geassed. Euphie was trying to help the Japanese, Zero manipulated her and painted this false story she was a lier and a murderer.

"but you told us that she betrayed us!"

"no, it was really me using Geass"

*kallen cries*