In other words, how dangerous could an out-of-control mad House Elf be?
There is no canon confirmation either way, in the books, Pottermore or interviews.
All we DO know is that:
Wizards suspected Winky of casting Mordmorde spell, which is a pretty dark and powerfull spell even if not an Unforgivable. Therefore, it's not impossible for a house-elf - at least with a wand - to cast known powerful Wizarding spells. This means that Unforgivables are at least plausible.
House elf magic is pretty powerful in general (apparating in no-apparate spell, blasting back malfoy, intercepting Harry's mail by Dobby, tracking down and taking down a fully trained Order member Mundungus, surviving drinking Voldemort's cave potion). This would mean that they would definitely have enough power.
Specific to Unforgivables, house-elves clearly are able to feel the strong negative emotions required to power Cruciatus and Avada Kedavra (witness Dobby attacking Lucius. Or Kreacher's emotions about mudbloods and traitors - if he was ordered to by Bella or Narcissa I have little doubt he could have Crucioed someone).
So, the PotterBusters rates this theory as "unknown, but not implausible".