Indeed they were.
After studying the teeth of the Neanderthals, Paleobiologists had come to the conclusion that they suffered periods of starvation and may have supplemented their diet through cannibalism.
Signs of cannibalism are defleshing, dismemberment, and skinning. Evidence of cannibalism also include cut marks made by stone tools used to remove flesh.
The manner in which the fossil skeletons were broken apart also shows this.
Bones and skulls were observed and was said that they have been smashed open to get at the marrow and brains inside.
The skull remains of younger Neandertals also bear incision marks, which are said to indicate that the victims were skinned.