harry potter - To what extent did members of the Order of the Phoenix use Unforgivable Curses against Death Eaters?

It seems that by the time of the Deathly Hallows, there is clear evidence that at least some order members and order allies using lethal force:

"I shall expect you and the Slytherins in the Great Hall in twenty minutes, also," said Professor McGonagall. "If you wish to leave with your students, we shall not stop you. But if any of you attempt to sabotage our resistance or take up arms against us within the castle, then, Horace, we duel to kill."

‘Harry, the time for Disarming is past! These people are trying to capture and kill you! At least Stun if you aren’t prepared to kill!’

However, many people on here (Why doesn't the Order use unforgivable curses in case of life threatening situations?) have talked about the Order being above using Unforgivable Curses, and there is evidence of this as well - In the battle in the Department of Mysteries at the end of OoTP, as far as I can remember (don't have my book with me) none of the Death Eaters were killed, although many were captured. So my question is:

1. When did the Order (and Order allies) first begin using Unforgivable Curses?


2. How widespread was the use of Unforgivable Curses amongst Order members and their allies in OoTP, HBP and DH, as far as we can tell?

Answer

First you must understand that Unforgivable Curses are three of the most powerful and sinister spells known to the wizarding world. They are tools of the Dark Arts and were first classified as "Unforgivable" in 1717, with the strictest penalties attached to their use. The three curses consist of the Killing Curse (Avada Kedavra), Cruciatus Curse (Crucio), and Imperius Curse (Imperio).

Now all three curses require great skill and power to cast. For example, Harry Potter was unable to effectively cast the Cruciatus Curse on Bellatrix Lestrange in 1996. Despite being furious with her for her murder of his godfather, he lacked the desire to cause pain for its own sake. Harry's "righteous anger" only inflicted a brief moment of pain on her.

Now to answer your questions:

1. 
   

   Aurors were permitted to use them during the First Wizarding War, and during the height of the Second Wizarding War under Lord Voldemort's regime, the curses were made legal, though this was presumably repealed following Voldemort's demise. As for Order, since they were run by Dumbledore he would never give green light to use Unforgivable Curses. You also need to understand that a simple cutting curse can be used to kill someone. So why would you, as follower of light, use Dark Arts when you can fight with normal curses and spells as effectively?

   
   


2. 
   

   Shortly before the Battle of Hogwarts, Harry not only successfully used the Cruciatus Curse against Amycus Carrow, he also cast the Imperius Curse against two individuals during the Gringotts heist which led to Harry, Hermione, and Ron getting in the Lestrange vault. Soon after being "Cruciated" by Harry, Amycus Carrow was also subject to an Imperius Curse cast by Minerva McGonagall. Perhaps because the curses were cast during a period when the ban on their use had been lifted, there is no indication of either Harry or McGonagall being punished for using them in these circumstances, but that was more of an exception than a rule. As you mentioned OotP, DA and other allies usually look down on Dark Arts and in my opinion would not use Unforgivable Curses.

   
   

Most of text taken from HP wiki on Unforgivable Curses