Congratulations on a fine kill. Good shooting. Considering where you hit him I have no doubt that he didn't even have time to feel anything. The twitching was just involuntary muscle activity, happens all the time.
I know what you mean about getting nervous,,,,, I've been hunting my entire life, and have killed more game animals than I can count, but to this day when I put the scope on a squirrel, rabbit, pigeon, or anything else for that matter, I still feel the adrenaline rush and can feel my pulse quicken.
Anyway, great lookin rifle, great picture, and great shooting. Congrats :)
ps (they make great stew mmm mmm)
Jeff