Gain control of target creature until end of turn. Untap that creature. It gains haste until end of turn. Add two mana of any one color. (The creature can attack and {T} this turn.)
Gaining control of a creature doesn't cause you to gain control of any Auras or Equipment attached to it.
If the target creature is an illegal target by the time Traitorous Greed tries to resolve, the spell won't resolve. You won't add two mana.
Traitorous Greed can target any creature, even one that's untapped or one you already control.