I don't think evilness defines whether he would allow himself a giggle-kicking fit.

His biggest concern is likely the concern that investigators are on his tail, that his file is open and hot, and that he's surveillance aware but knows he can't be perfect, so his freedom and how long it lasts depends a good part on luck. While this stressor might be crushing (it would me) he may be able to manage it and push it out of the way long enough to enjoy his newfound notoriety, and even express some energetic glee over it.

Another concern (which is where we wonder about evil) is coming to terms with recognizing he took another human life with planning and malice. He committed murder. The capacity to commit murder doesn't come to everyone, and many people can't. But then we also have a robust military in which a lot of people do kill, or find at least they have the capacity to suppress their empathy enough to kill. Our assassin didn't hesitate. It doesn't make him evil (which is a judgement usually appointed to fictional persons by the author or readers) but it does put him in the same category as all whodunnit culprits, someone with the capacity to kill if circumstances warrant it.

But again, it's a matter of whether he's able to manage his moral concerns, or is already good with it. Then it's a matter of whether it's in character for him to indulge in a giggle and a kick. (I would.)