My man, they treated him as a gopher and tied him down with tons of menial tasks. Treating him as a servant rather than a team member. They actively TURNED him into a liability, and the Knight/Priestess very much did so intentionally. The Priestess was just more outspoken and blatant about it. The rest of the group went along with it or didn't care, which is just as bad. Sasha standing by and permitting it in itself makes her irredeemable.
Hyce BECAME a liability not by accident but by intent. He was never trained, never had any lessons, and they loaded him down with so many tasks he wouldn't even have time to train on his own. They also ensured he wouldn't get any experience nor fight the initial easy enemies so he'd fall back further and further. Sasha even fucking NOTICES this when they recruit new people, and do all these things with them. The same things that were done for her, while she received training Hyce was basically send to do chores and clean up for the party.
No, Sasha was a horrible, selfish, egotistic person long before that. She has a tendency to just ignore things that are inconvenient to her. To handwave them away. To ignore how others are treated and used, as long as the people doing it benefit her personally. She saw what her hanger ons did, and allowed it. She never even saw a problem with it and just went along with whatever bad excuse they gave her.
Hyce "understanding" is part of the author's endless glazing of Sasha. Something even the Japanese audiences is furious about and complains about endlessly. One of the most chill audiences who usually hardly ever speak up about anything. Sasha is a horrid person, in her own way she's more evil than many selfish and intentional evil parties kicking out people. And I doubt there'll be any comeuppance for the people who intentionally sabotaged Hyce this time around either.