I have the same issue, though I've never had homicidal thoughts.
I don't have many emotions: contentment, irritation, discomfort, and nothing. I feel nothing the majority of the time.
I display emotions in conversation, but I am only mimicking what I've learned to be appropriate. Usually, you are meant to mimic the speaker's expression. They find it very rude if you do not.
I can cut my emotions off as well, though I'm probably just very good at ignoring them.