What is a word for a person who constantly speaks negatively? [duplicate]
In British English, "Naysayer" is common for someone who says that something is not possible, won't work, shouldn't be tried, etc.
killjoy
noun
a person who deliberately spoils the enjoyment of others.
"a few killjoys try to reform the seasonal activities"
synonyms: spoilsport, moaner, complainer, mope, prophet of doom, Cassandra,
Jeremiah, death's head at a feast
Consider this possibility (definition from Oxford Dictionaries online):
curmudgeon /kərˈməjən/ noun A bad-tempered or surly person.