No one thinks AVALANCHE are blameless, least of all AVALANCHE themselves. But there is a difference between accidentally killing bystanders as a regrettable consequence of a bombing and intentionally pressing a button to annihilate a town in full knowledge and intent of the consequences. Cloud handed over the black materia with a broken mind, Reno pushed that button with a sound one.
Dirge leans pretty heavily towards a sympathetic depiction of Lucrecia, TTM, but you're happy to hold her in enormous disdain with no issues. Why is it such a problem when I take note of other characters much greater sins? But the fandom works so much harder when the Turks reputations are at risk.
We have two depictions of the pillarfall, both of which have ample opportunity to show pressure being applied to the Turks in cutaway scenes (the way they do with Reeve.) But they don't. Why not? Neither version has any indication that 'feeling under threat' is any part of the Turks motivations, and both have many things that work against it.
If they truly are under such great threat, it doesn't even make sense to use the Turks for this mission. But between 'maybe the Turks are not on as thin ice as we think' and 'Shinra Exec decisions make no sense' we pick option A.
'It's suicide to stand up to Shinra' But there's a bunch of thriving resistance movements. Fuhito's AVALANCHE was only stopped by the Turks themselves.
'Someone else would do it' Who, though? The Turks are the only ones that are trusted with Shinra's deepest secrets, on multiple occasions we see SOLDIER intentionally kept away from Shinra's darkside in both CC and BC. The Shinra military are only show to accomplish things by leaving very indiscreet giant piles of corpses behind. We know of no one else with the appropriate skillset, and while it's probable such people exist, it's likely they would be at the very least less effective.
'They're under threat' From who? How? The military are not a match for them except pinned down at overwhelming odds, which is very difficult to do. They have the skills to disappear, which we know because most of them do. There's so many things that don't work, like Scarlet trusting someone she hates and wants to kill with her personal security in Gongaga, despite a significant risk that he will just kill her... IF she doubted his loyalty.
'they're indoctrinated from a young age to be evil' True for Cissnei, but the others have all kinds of varying backgrounds outside of Shinra, like being champion hunters in Mideel, working for Midgar gangs and being top military cadets
Post Meteorfall, there are changes, but there's also other things. The AVALANCHE crew change their ways, Cloud becomes a delivery driver, Barret goes drilling for oil, Tifa goes back to bartending.
The Turks are still the Turks. Rufus puts a Shinra logo on the memorial to the meteorfall dead. When this is criticised, his response is not 'that's fair, I'll remove the logo' its 'send the Turks to silence my critics'. They want to be redeemed, but don't want to change. Rufus wants to be trusted, but he doesn't want to stop lying. When meds are stolen, they go right to punishment beatings first and ask questions later. When they feel Rufus needs to talk to Evan, they could ask and then accept a refusal, or put him on the phone. Instead they violently kidnap him. They're building a monument...but they're also forcing people to work on it against their will.
What am I leaving out?
I don't dislike them, by the way. I think there's a writing problem because they have to play conflicting roles that don't fit together.
Edit: The execs are repeatedly shown not to have that kind of power, as shown by the means they use to deal with AVALANCHE. They either ineffectually throw the infantry at them or use giant robots. Super black ops people are not in evidence, the only exception is DG, which come with a host of other problems.
