Yes, FATALIST is a Scrabble word!

Letter values in Scrabble US: F 4, A 1, T 1, A 1, L 1, I 1, S 1, T 1.

Is it a valid word?

FATALIST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

anyone who submits to the belief that they are powerless to change their destiny

Also: predestinationist.