Was Khaine getting pissed about Freddy telling story about how he killed one of his Chosen?
While I think it´s one valid interpretation, we actually don´t know to what degree the Khaine in the quest actually gets angry when such a prominent worshipper is killed in the way he did.
I mean, while Tullaris might have died an unworthy dead by Druchii standards, that mainly is due to the Druchii´s superiority complex doing mental gymnastics with everything to make it fit their own twisted worldview:
nd if you lose, in Druchii society, it means that you are weak. And if you are weak, then you should be dead. Malekith has been repulsed from Ulthuan multiple times, but it is his armies which are so repulsed, and for all his might the twisted arrogance and pride of the Druchii know that it is only that the Asur are elves - weaklings that they are, they are still elves and have the support of the damned Cadai - that they are kept back from their true destiny. Just as they also know that they will, inevitably, fail in this course so that the Druchii will reach their true ascendance.
So, no. Not the same at all.
He lost. So how Chosen was he, really? How Chosen could he have really been? Hellebron has raised up many champions for Khaine, not all live that long. He was only one of the First Executioners. And besides, Hellebron has ever been weakening in these years, she must bathe in more blood every Death Night to regain her youth, faltering as she is compared to glorious eternal Morathi, perhaps she made the wrong choice all those years ago, etc. etc.
Blackout made a brilliant post on their quest about how elves can basically mentally rejigger anything that happens into an elven or at least personally positive version of just about everything, or at least negatives for the other party and not them. It applies here, too.
While they see all humans basically as cattle and consider dying to them ---no matter their skill-- to be an humiliating death, Khaine seems to think somewhat differently. Frederick´s scars of the Khainite sigils carved in his cheeks reacting to Khaine´s power during the fight against Tullaris, heavily implies that human Khainites worship the same Khaine --just in a different way--, and, that if they´re good enough at murdering, Khaine very much is willing to provide them with blessings. And he considered Frederick a good enough murderer to try and make a play for his soul once he got the opportunity.
Tullaris actually did not got out like a scrub just because he fought against a human. He died in a very high-level and brutal fight, during which both parties came very close to dying and, also important, were utterly full of hate towards each other. Then there was the way it ended, with one party getting murdered in a horribly gruesome and ironic way. I.e. this normally would be precisely the sort of fight that Khaine favors.
Might Khaine be angry with Tullaris because he died? Perhaps. But, considering in just what kind of fight he died, he might not really care that much, and just focus on other tools to generate more murder, instead.
Gwendolyn´s reactions might not have been caused by Khaine´s anger, but by just how intimately and full of hate Frederick was recounting the bloody and gruesome end to the above fight.
Also, I feel that Hultressa´s "indescribable expression", might´ve just been due to seeing that disturbing/weird scene of Frederick gently comforting Eldyra, while hatefully recounting in exact detail how he killed a figure that she, as part of Druchii society, would´ve consider very, very dangerous.