So it should be settled that the reason for the psychic damage is because she is taking out members of FILTER (possibly friends) which she used to be part of. While it is for the better overall, it does not make it easy to do.
I think KNYFE and Choke were close. Very close, once upon a time. Maybe they were both on the Special Operations unit and went on countless missions together during KNYFEs decade+ in FILTER, a large measure of trust was built between them. It's hard to get close to people when you are an agent of FILTER, but there was probably a great deal of compfort in that relationship.
It must have felt like the worst kind of betrayal when KNYFE went rogue. And Choke knows full well how betrayal is repaid in FILTER.
That's my guess. The psychic damage is that mental toll it takes on you to kill someone you once considered your closest friend.
Nah, KNYFE wanted to leave, and saying goodbye to friends got her all choked up.
Literally.