Yes, KILLER is a Scrabble word!

Letter values in Scrabble US: K 5, I 1, L 1, L 1, E 1, R 1.

Is it a valid word?

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

someone who causes the death of a person or animal
the causal agent resulting in death
a difficulty that is hard to deal with
predatory black-and-white toothed whale with large dorsal fin; common in cold seas

Also: cause of death, killer whale, sea wolf, orcinus orca.