People have wondered what was up with Barriss' reaction, exactly what was so disturbing... And here you'll find out. I'm drawing a bit from the original Thrawn Trilogy, how Legends treated the Clone Wars before the prequels, some lesser known Legends material, and a deleted scene from RoTS.
by lord Martiya
Year 16 of the Empire: The truth about Baarriss Offee, part 2
The Lady Pellaeon, as she went by these days, hadn't expected it would happen so soon. And yet here she was on Malachor, alongside Ahsoka and the infamous crew of the Ghost, that included Deepa Billaba's final Padawan Caleb Dume (or Kanan Jarrus, these days), and this Force sensitive she hadn't known before, Ezra Bridger. And aside for Ahsoka and Ezra they were as wary of her as they were of the other guest, Darth Maul. Who had decided to actually help rather than try and steal the holocron (at least for now) only because he wanted to enjoy the mess from her presence-and he didn't even know what Kanan and Ahsoka knew. Even the Inquisitors blocking their way were wary of her-but she could use it. So she took a comlink from one of the Inquisitors and made her communication.
"Seventh Sister, I am Baarriss Offee. I'm here to meet you." she said. Then, seeing everyone but Ezra (too young to know) and Ahsoka was even warier of her, she explained: "Already dealt with that, I'm safe."
"Wait... Is this Seventh Sister...?" Maul whispered. Then, when Ahsoka nodded, he laughed and complimented Dooku and the Twillers for still being able to mess with the galaxy even after so long.
"You... You're crazy, you know?" one of the Inquisitors said. "She's gonna kill you."
"Or I will kill her. Now, could you lead us to her? Or should I kill myself and let you deal with her?"
After a shout of anger, the Inquisitors told the group to follow them, while Maul admitted his reluctant admiration for taking herself hostage.
"I don't get it, why are they all like, well, this?" Ezra asked as they reached where the Seventh Sister was, showing again how young he was.
"Why are the Clone Wars called that?" she asked back.
"Because the Republic used a massive army of clonetroopers to fight it."
"Correct, but insufficient-both Republic and Separatists used cloning technology to fight the war."
"What?"
"The Republic most famously used Kaminoan-made clones of Jango Fett as soldiers and the elite of the Grand Army, but also Spaarti cloning cylinders captured at Cartao to create a supplementary army from other sources. It was faster, only three years, but their debut at the Battle of Coruscant showed they weren't anywhere as good as the proper Clonetroopers. And of course the Khommite soldiers were all clones, but they do use cloning as their only mean of reproduction, so it would be unfair.
"On the Separatist side... They rushed Spaarti clones to create a wave of infiltrators, created too fast to not suffer from Clone madness. My husband-"
"Your WHAT?!" Kanan shouted.
"-had dealings with them, he knows how psychotic they were. They were all identified and dealt with quickly, but the effort kept the Republic busy long enough we couldn't take advantage of Sev'rance Tann's failure at organizing a unified military before Grievous took over and forced his order of battle on the Droid Armies. There's also the Morgukai Shadow Army, created with the Spaarti cylinders that were brought to Felucia before the Battle of Cartao-and the second wave of infiltrators, ones that didn't suffer from Clone Madness, aside for homicidal reactions to their originals or being exposed as clones, and believed themselves their originals until their programming was activated. Before they could be activated, however, they were discovered by accident during a high-profile trial due their quirk about adding a vowel to their originals' names."
By then they had reached the seething Seventh Sister, who was barely keeping herself from jumping. And by seeing her face, effectively identical to hers aside for the different tattoos and eyes, he realized what was happening.
"Wait... The Seventh Sister is your clone!" he said.
"No, I am hers." she admitted. "That's why I wasn't tried and executed as a traitor after my betrayal of the Order: they didn't know how much was me and how much the conditioning until my original was found on Felucia and discovered almost as much in doubt about the Order as I was. Though I wonder how did you survive, Barriss."
"Don't you dare to use that name!" the former Barriss Offee spat. "I am the Seventh Sister now, far greater than the little girl captured by the Separatists and replaced by you!"
"You were a Knight. And even had a Padawan after you were rescued. And from what I've been told, you Inquisitors aren't the best fighters." Dark Siders really loved Dun Möch, as they called that way of taunting to break someone's concentration and ability to think straight, but, quite ironically, they often were far more vulnerable than Light Siders. It was just their good luck most didn't use it... And the Seventh Sister's horrible luck that her clone had come ready to do just that, to a point nobody expected.
And when the Seventh Sister lunged at her in anger, Baarriss used the Soresu skills she had refreshed ever since Tarkin had shown his knowledge of her continued survival and the lightsaber provided back then to block. She was no Obi-Wan Kenobi, and her Soresu wasn't anywhere as good at the one of the man who put Grievous down, but her opponent was no Grievous and it was enough to hold her own until the Seventh Sister noticed the lightsaber: hers.
"You THIEF!" she screamed, her growing anger pushing her to do the stupidest possible thing against a practitioner of the Soresu lightsaber fighting style: spin the blades of her lightsaber to try and chop her down better.
Just as Grievous in his final duel lost two hands and weapons to parries at slightly different angles, the first attack with the spinning lighsaber saw its emitters cut away by Baarriss' block. The Seventh Sister could only look in horror and anger at her wrecked weapon, especially as Baarriss' use of Soresu came from her to begin with. Then her look turned to surprise as she felt her heart pierced by what had once been her weapon.
"For what is worth, I'm sorry it had to end like this." Baarriss said to her original. "I hope you'll find peace now."
"See?! Neither of the veteran Jedi hesitated!" she heard Maul shout, and as she looked around she saw that during her duel with Barriss the others had taken down the remaining Inquisitors-and Maul had apparently tried to have Ezra take down one of them while he was incapacitated, and was now scolding him using Ahsoka's use of lethal force and Baarriss finishing off a disarmed foe as example.
"That's because we're weak." Baarriss replied. "A true Jedi would have given them mercy."
"No, a strong Jedi would have done so. Because someone like Kenobi would have been strong enough to have mercy without it backfiring on him. I know that from experience. And until then... Until then, we can't afford any mercy. Now let's move, I have a bad feeling about this."
Maul's fear was proven right: after they had found the holocron and were about to leave they found Darth Vader himself standing on his slowly descending fighter.
notes
So, how did I come up with that? A few possibly disconnected parts from Legends.
First, we have a dialogue in the original Thrawn Trilogy, where Pellaeon revealed that the enemy, whoever they were (as the Separatists had not been conceived yet), had used Spaarti cloning cylinders to create infiltrators. The dialogue was more about Pellaeon finding out Joruus C'baoth, being an early clone, was psychotic even before Dark Side brain damage settled in and being pissed that Thrawn exposed his crew to him, but it also explained the enemy eventually solved the Clone Madness issue and that they had a quirk about using the same name as the original but with an added vowel (Joruus C'baoth was the clone of master Jorus C'baoth, and Luke's clone from the trilogy called himself Luuke).
Second, we have the fact that Barriss Offee was originally supposed to be shown killed by the Clonetroopers on Felucia, and other material created before TCW showed her as having a Padawan by the end of the Clone Wars.
Finally, in Legends the Separatists creating the Morgukai Shadow Army, cloned from members of the Morgukai secret society of the Nikto species, on Saleucami using Spaarti cylinders. That is, we have confirmation that the Separatists maintained rapid cloning capability even after losing Cartao.
And when one takes into account that Barriss Offee was exposed as a traitor and imprisoned in "The Wrong Jedi", what other conclusion I could take that the traitor was actually one of the cloned infiltrators and was caught during booking as she absent-mindedly corrected the Clonetroopers that her name was Baarriss?