Well, first, the fat is not the problem regardless of how much carbs we eat. The problem is the carbs. Why would fat become bad once as we included carbs? See, it doesn't make sense. The fat still goes in fat tissue as normal, but the carbs shift the balance toward fat accumulation by stimulating insulin which inhibits fat release from fat tissue and by providing the substrate, glycerol, to form triglycerides for storage. Without the carbs, there is much less glycerol and there is much less insulin, so there is much less new trigs being formed and fat is allowed to flow freely from fat tissue. It's more complex than that and we can read about it in Taubes' GCBC but it will serve to answer your question at this time.