I had to go to quite a bit of therapy sessions after the Vegas shooting last year. We did talk about what I witnessed and how it made me feel but with so many appointments we eventually ended up on repressed memories I had, like being molested as a child and the abusive home life I had until I was kicked out at the age of 17. Was just a natural progression of things. Sounds like you should talk about it, though, IMO. I wasn't comfortable with it, myself, and it was physically painful to utter those words, but I felt better after awhile because it didn't have such a choke hold on me. Maybe it isn't like that for you, but it is for some others.
That being said, I eventually ended my sessions with that therapist after she made the legendary comment of "All gay, lesbian, and trans people were sexually assaulted or abused as children." For all the good she did, she still had that stupid fucking opinion. Had to call it quits there.