Does using {Sadistic Hypnotist}'s ability count as the death of a creature? And would this affect morbid?
Yes it does
Dies means goes from the battlefield to the graveyard. Doesn't matter how or why this happens.
Yes and yes:)
700.4. (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=700.4.): The term dies means "is put into a graveyard from the battlefield."
Also, dying is different than being "milled" ,by say {millstone}, because they are sent straight from library to grave yard. Sacrificing does send them from battlefield to grave, so it counts