Really I feel the slenderman might just fit into the scientific community as an animal or even myth as just another creature to be studied and nothing more.
As for the second question, considering the wacked up shit that happens in the bible, probably not. An all powerful entity would probably have enough on their hands, dealing with one moronic race at a time.
As for the last question, you will probably get a lot of different answers depending in who you ask.
You might not be able to tell, but I am a misanthropic pagan so I really don't care for the bible.