Scar became complacent, as he thought was well within his rights to. His plan had went through, the only people who could stand in his way were dead. Relax and act like a king, being waited on hand and foot. Also in the real world the lionesses do all the work too, so that's not so unexpected.
I am curious as to how you wanted him to act post-regicide. :3
I enjoy the Lion King. I never cried though. Not one tear.
The stage show was also pretty amazing.