He doesn’t get his power by killing people. He is powered by fear. He can only kill when someone is scared, because he becomes he physical manifestation of their fear - so he needed Henry because Henry can just kill people whenever.
So then why/how did he kill that little girl? He told her he could remove her birthmark, but had to count to three. She was totally soothed and not afraid and then he jumped at her and ate her.
Im not sure that I agree with this.
IMO, he becomes the manifestation of a childs fear because it makes them easier to kill. He burst Bowers out of the Psych Hospital because he already hated the Losers and he would add confusion and fear into them. The It films were horrible at plot and character and changed things whenever it suited the filmmakers.
Pennywise got his power from being an extraterrestrial almost immortal being. He killed because he needed to eat and humans became his food source.
The real terror in the book at least was the nature of the town and its people. And all the broken lives and unspoken misery.
Then why at the end of the first movie did they say that IT did not kill the girl because she was not afraid?
The movie sadly isnt anything like the book. Like Wookinpanub pointed out, fear makes Its food taste better. It doesnt kill because its evil or bloodthirsty. It has to eat. We are its cattle. However, in the book, it incites violence in other human beings. As I remember, the lumberjacks in the bar, the gangster shootout in the streets of Derry, The Black Spot fire and the explosion at the factory during the Easter Egg Hunt.
The movies were a massive missed opportunity to recreate the unspoken fear and dread that IT brought to Derry.
I guess we will have to wait for an extended miniseries to get a fair recreation of the novel.