I dont understand how its weird when ppl cheered for that particular stairs scene. Joker is one of the most famous and liked character ever, and he finally presents himself in that particular screen. Ofcourse ppl liked it when he comes on screen. This is what the whole movie was about, ORIGIN. And phoenix did an amazing job in the acting department. The walk towards the elevator was worth cheering. The transformation was amazing.
I remember ppl cheering for vader when he wrecked sh*t at the end of rogue one. Both are villains, both did the wrong thing and both showcased their ruthless nature. One with skill, one with overcoming his timid nature and going bat sh*t crazy.
And among the so called educated peeps, these same villains are decorated in their rooms, loved and praised, in regular chit chats.