Yes, ANTAGONISM is a Scrabble word!

Letter values in Scrabble US: A 1, N 1, T 1, A 1, G 2, O 1, N 1, I 1, S 1, M 3.

Is it a valid word?

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

a state of deep-seated ill-will
the relation between opposing principles or forces or factors
an actively expressed feeling of dislike and hostility
(biochemistry) interference in or inhibition of the physiological action of a chemical substance by another having a similar structure