Yes, DESERTER is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, S 1, E 1, R 1, T 1, E 1, R 1.

Is it a valid word?

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

a disloyal person who betrays or deserts his cause or religion or political party or friend etc.
a person who abandons their duty (as on a military post)