Imperio.

[pippin_999]

No: HPFGUIDX 177084

CJ:
If JKR intended us to  understand that only some faction or other of the 
WW considered them "unforgivable", then she did a very poor job of 
communicating it.

Pippin:
Sirius says that the Aurors were authorized to use the Unforgivable
Curses during the last Voldemort War, so the term wasn't always
taken literally. 

Sirius doesn't like that decision, but he says  "I'll say this for 
Moody, though, he never killed if he could help it. Always brought 
people in alive where possible. He was tough, but he never 
descended to the level of the Death Eaters."

It's implied that Moody might have used Imperio and Cruciatus
without descending as far as a Death Eater. Also, Slughorn
says that murder is the greatest evil, implying that torture and
magical coercion aren't  as  bad.  

 Snape, who is very powerful and very well disciplined, seems to be 
able to use the Confundus Curse with as much effect 
as Imperio. Perhaps that is one of the reforms that Harry was able
to implement in later life.

I don't think there is much excuse for using Crucio, but after 
seeing how easy it was for Harry to use it, I can appreciate 
Crouch's dilemma better. A prohibition is only as good as 
the willingness to observe it. If Crouch hadn't 
authorized the use of the Unforgivables, he might have had to 
put his best fighters in Azkaban, and then what would
have happened? 

The solution, in the real world, would lie in  training and esprit
de corps, but those can't be implemented overnight.

Pippin
thinking that Harry had to find out that he could use
Crucio to understand why the mere prohibition was not enough.