Yes, MADNESS is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, D 2, N 1, E 1, S 1, S 1.

Is it a valid word?

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

obsolete terms for legal insanity
an acute viral disease of the nervous system of warm-blooded animals (usually transmitted by the bite of a rabid animal); rabies is fatal if the virus reaches the brain
a feeling of intense anger
the quality of being rash and foolish
unrestrained excitement or enthusiasm