Uhh, Im not familiar with scary seeds. I guess you could find some really weird and unnatural features in a seed like a weird cliff or broken up mountain. There are no special seeds with certain entities either. They're all the same like find three wither skulls to create the Wither, or find the End Stronghold to go to the End Dragon.
Things that can make your world "scary" is finding texture packs online or on this website and download them and put them into your Resource Packs folder in your minecraft folder.