During fermentation, it’s normal for mead to produce some off-putting odors and flavors, like sulfur or “rotten egg” smells.  However, these typically dissipate as fermentation progresses.  It has likely gone bad if the mead smells excessively foul, has mold growth, or exhibits signs of contamination, such as unusual colors or textures.  Trust your senses — if it looks, smells, or tastes spoiled, it’s best to discard it and start anew.