Denatured means messed with, not natural. Yucky.
From wiki:
...is ethanol that has additives to make it poisonous, bad-tasting, foul-smelling, or nauseating to discourage recreational consumption. It is sometimes dyed so that it can be identified visually. Pyridine, methanol,[2] or both can be added to make denatured alcohol poisonous, and denatonium can be added to make it bitter.
Denatured alcohol is used as a solvent and as fuel for alcohol burners and camping stoves. Because of the diversity of industrial uses for denatured alcohol, hundreds of additives and denaturing methods have been used. The main additive has traditionally been 10% methanol, methyl alcohol, giving rise to the term "methylated spirits". Other typical additives include isopropyl alcohol, acetone, methyl ethyl ketone, and methyl isobutyl ketone.[2]