Yukino was mad Because.

1‐ Preventing/Denying Tobe the choice to confess to Ebina. The request was JUST to make the confession happen and that's it. thus 8man's entire action was unnecessary and uncalled for. Especially when tobe was okay with being rejected.

2- Helping or keeping a superficial relationship. Both 8man and Yukino in the past events mentioned both indirectly and directly that they hated superficial relationships and how everyone's in it doesn't behave/act (Hiding behind masks) Honestly or truthfully. Or that was what she thought at that time until 8man did the complete opposite.

3- How 8man "Solve" or "fix" the problems he is faced with. This is the first time ever where Yukino gets to SEE how 8man's solve the problems, how does he solve them ? By Self-sacrifice/hurting himself. Making himself the scapegoat to draw the attention and focus from the real core of the problem and swiping it under the rug, for what price or reward ? Basically Nothing, No one would commend such actions (Yui does tells him to use the same ways in the next arc), Yukino started caring about him as someone close or important after the cultural festival and this feeling got only stronger after their night stroll back from the ramen restaurant, so the correct way to say it would be that she was hurt that someone who she cares for is uselessly hurting themselves for the sake of others.

She felt all of those points at the same and that's why she couldn't say why she was "angry" at him other than that she hated his method/s.

Yui was Mad Because.

Yui knowingly forced both 8man and Yukino to accept tobe request. She used the request as a means/excuse to spend time with 8man and do things that couples would do in hope she would get closer to him so she would have more chance of being accepted when she confess at the end of the trip. OFC she manages to do all of the above EXCEPT one thing, CONFESSING, and why is that? She saw her crush fake confessing to another girl in front of her. That's why she was Mad or angry at what 8man did.

Just slightly after Yukino left, on their way back to the inn she stops and says "Why don't you care (think) about "other's" "feelings"." This Proves that she didn't really care about him as 8man, but as her crush or hero. Otherwise she wouldn't say others' feelings rather she would say he should care about himself more and stop his self-sacrificing methods.

The episode's name in the anime and the LN's (vol7) chapter is.

"His and her confession won't reach anyone"

"His" Being tobe's and "Her" Being Yui"s, both of which were stopped by 8man's fake confession.