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

Question

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 1

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of the Second Wizarding War under Lord Voldemort's regime, the curses were made legal, though this was presumably repealed following Voldemort's demise.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

Answer 2

The Order seems to have used Unforgivables only very sparingly.

It’s very rare that we see a member of the Order use any Unforgivable Curses. We only know of a few occasions on which they did, and although there may be others we simply didn’t see, but it certainly seemed against their moral code. We don’t know how many members of the Order used Unforgivable Curses during the first war against the Dark Lord, but presumably it was also sparingly, if ever. Barty Crouch Sr authorized the Aurors to be allowed to kill, as well as use the Unforgivable Curses, and Sirius Black, a member of both the original and the restarted Order thought he went too far.

“Crouch’s principles might’ve been good in the beginning – I wouldn’t know. He rose quickly through the Ministry, and he started ordering very harsh measures against Voldemort’s supporters. The Aurors were given new powers – powers to kill rather than capture, for instance. And I wasn’t the only one who was handed straight to the Dementors without trial. Crouch fought violence with violence, and authorised the use of the Unforgivable Curses against suspects.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 27 (Padfoot Returns)}}

It seems unlikely, then, that the Order as an organization would use Unforgivable Curses in anything but the most extreme circumstances. (The Order also wasn’t part of the Ministry, so it’s unclear if the permissions applied to them, though even if they did it’s unlikely the Order would approve morally) Moody, who was both an Auror and Order member at the time, tried very hard to avoid killing, and didn’t “descend to the level of the Death Eaters”, which would imply he didn’t often use Unforgivable Curses.

“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.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 27 (Padfoot Returns)}}

Overall, it seems like a reasonable conclusion that the Order of the Phoenix didn’t use Unforgivable Curses unless absolutely necessary.

There wouldn’t be any time that the Order ‘started’ using them.

There doesn’t seem to be any time when the Order of the Phoenix really “started” using the Unforgivable Curses, they always considered it wrong to do so and would have avoided it. There were other spells not connected with the Dark Arts they could use, and they used those instead.

There are four known cases of Order allies using Unforgivables.

In the second war against the Dark Lord, we only know of four times someone associated with the Order of the Phoenix ever used an Unforgivable Curse. Harry attempted to use Crucio in the Ministry of Magic, then again in Hogwarts, and Imperio in Gringotts. McGonagall used Imperio on Amycus Carrow at Hogwarts, and Snape used Avada Kedavra on Dumbledore like they’d agreed upon. Three of those times were Harry, who even Dumbledore says had a hot head. It’s also unclear if he was ever made an official member of the Order - if we don’t count him, then there’s only two instances known. In McGonagall’s case, the Unforgivable Curse used was the least “harmful” and also only used to make Amycus sit quietly so he could be captured. In Snape’s, the Killing Curse was the best way to give Dumbledore a quick and painless death.