Yes, ASSASSINS is a Scrabble word!

Letter values in Scrabble US: A 1, S 1, S 1, A 1, S 1, S 1, I 1, N 1, S 1.

Is it a valid word?

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

a murderer (especially one who kills a prominent political figure) who kills by a surprise attack and often is hired to do the deed
a member of a secret order of Muslims (founded in the 12th century) who terrorized and killed Christian Crusaders