Cassian is an apprentice under the Sith order. He was trained under dark side practices since he was born. However he is dating a Jedi. Like him she was born and raised under the light side practices. Of course because you two serve opposite sides your relationship cannot be public.

You had snuck out late one night and hopped into an aircraft. You wanted to see Cassian. You flew all the way to where the Sith were currently hiding at. You had no problem killing other Sith members but Cassian had made you swear if he told you where he was you wouldn’t kill anyone at the location mindless provoked.