Yes, OVERKILLS is a Scrabble word!

Letter values in Scrabble US: O 1, V 4, E 1, R 1, K 5, I 1, L 1, L 1, S 1.

Is it a valid word?

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

the capability to obliterate a target with more weapons (especially nuclear weapons) than are required
any effort that seems to go farther than would be necessary to achieve its goal