Actually, it is much worse than either sugar or HFCS. Sugar and HFCS are nearly identical. They are both about half glucose and have fructose. Agave is mostly fructose. Both/either are a nightmare for diabetics, but fructose is much worse and not just for diabetics but for everyone in anything but tiny amounts.
If you care for a detailed (VERY detailed) explanation, you can view it
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A hepato-toxin similar in harmful effect to alcohol which among other things causes fat deposits in your liver. Your meter can't even begin to tell you the evils of this stuff.
Fructose, the consumption of which has skyrocketed over the last 40 years is one of the prime suspects in the diabetes epidemic we are now in. I can't imagine anything more ill-advised than INCREASING our consumption of it.