Yes, SPOILER is a Scrabble word!

Letter values in Scrabble US: S 1, P 3, O 1, I 1, L 1, E 1, R 1.

Is it a valid word?

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

a candidate with no chance of winning but who may draw enough votes to prevent one of the leading candidates from winning
someone who takes spoils or plunder (as in war)
someone who pampers or spoils by excessive indulgence
an airfoil mounted on the rear of a car to reduce lift at high speeds
a hinged airfoil on the upper surface of an aircraft wing that is raised to reduce lift and increase drag