Some people think agreeing to nothing is a license to criticize everything.

There are people in this world who won't agree to anything or offer an opinion to anything when asked or to anything brought to them. But at the very first chance, they will offer criticism of other people's answers to the question asked. They will deride things that have been offered to them that was taken back for lack of an response. To them, their derision and unkind words are an expression of superiority. They feel empowered in their ability to thwart the efforts of others while offering nothing themselves.
They place a high value on their response when in reality they can offer nothing of substance. They inhabit meetings purely for opportunities to not participate rather than working together for a common outcome. They do so thinking what they do is invisible and so they do it in plain sight. These people feed on the civility of others and polite manners, thinking that there is no word for what they do. In fact, there is one word that describes them: Spineless.