Would Defiant Dance kill units with two might, due to the wording not saying to a minimum?

No, Defiant Dance would not kill units with 2 Might. Defiant Dance gives one unit +2 [M] and another unit -2 [M] this turn. The -2 [M] is a Might reduction, not damage. According to Rules 708-710, a unit's Might is simply reduced by 2. A 2 Might unit would be reduced to 0 Might, but reducing a unit's Might to 0 does not kill it. According to a rule, "If a Unit ever has nonzero damage marked on it equalling or exceeding its Might, it is Killed." The key word is nonzero damage. A unit with 0 Might would need at least 1 damage marked on it to be killed. Simply reducing a unit's Might to 0 through Defiant Dance's -2 [M] effect does not kill it—the unit remains alive on the board with 0 Might and 0 marked damage. The unit would be extremely vulnerable (any damage would be lethal), but it survives the Might reduction alone.