This is mostly a copied scene from Hibike! Euphonium Season 2 Episode 11 "Hatsukoi Trumpet" with some dialogue changed. You might want to rewatch it first. Or after reading this, replacing the dialogue with that from this story in your head. Or have it side by side. Anyway, it goes from 07:17 until 11:50.

Yakusoku no Élégie

(Hibike! Euphonium, death, consensual)

At that evening Kumiko felt almost choking under the weight of loneliness. Ever since her elder sister knocked heads with their parents and moved out to live her own life, the atmosphere at home was rather detached. Or maybe it was always that way? As if they weren’t even a family, but just some strangers who happened to live under the same room and only tolerated the presence of each other because they had no other place to go.
Kumiko’s father was working late as usual, though she wouldn’t be surprised if on that day it came not out of a necessity, but rather out of being not so eager to come home. Kumiko was leisurely eating her supper in a silent company of her mother who in spite of sitting at the same table looked distant and unreachable. Kumiko couldn’t really blame her though as her own thoughts were also far from home. What bothered her the last few days was how she was seemingly avoided by Reina, her… bandmate? Friend? Beloved? What exactly do these words mean anyway? Kumiko couldn’t just put a label on Reina like that. Reina was Reina. She was special, different from anyone else. Ah, what an exercise in futility, trying to put feelings into words. Maybe she could try expressing them with music instead? Kumiko wondered what kind of piece it could have been…
As if waiting for Kumiko to think of music, a melody sounded from her phone signaling an incoming text message. Oh, and it was from Reina too. Finally a reply after several days of seemingly unprovoked ignoration. Not quite the reply Kumiko hoped for though, as it said: “I am going to kill you”. Before she could even comprehend it, the second message appeared on the screen: “Can you come out right now?” Well, at least Kumiko would have a chance to talk to Reina before being killed.
“Sheesh, Kumiko,” her mother sighed, “Doesn’t dad tell you not to play with your phone over dinner?”
“Yeah…” Kumiko replied absentmindedly as she typed “Okay” and sent it back.
She wondered if she should forego dinner and come out right now just as Reina asked. Reina wasn’t a kind of person to joke about killing people, so Kumiko would most probably not need that supper anyway. But if she just dashes away in the middle of eating, it would look suspicious. So for now Kumiko has just put her phone aside and returned to her food as if nothing happened.
“Thanks for the meal”, Kumiko recited when she finished.
“You can go, I’ll wash the dishes”, her mother commented.
“Thanks”, Kumiko said, rising from the table. “I think I’ll go for a walk”.
“At this hour?” mother sighed. “Try to come home before dad does if you don’t want him to get mad”.
“Sure”, Kumiko replied as she headed out.
She knew she wouldn’t come home before dad or ever, but it’s not like she could tell her mother she was going to be killed.

Kumiko ran to the meeting place to compensate for the time she spent finishing her dinner, but of course Reina was already there waiting. Since she told to come ‘right now’ she had probably already been there when she sent the text.
“Sorry”, Kumiko apologized as she tried to catch her breath. “I’m late”.
Reina didn’t reply and just headed away. What, did she call for her just to ignore her face to face? Or did she mean she wanted Kumiko to follow? Thankfully, Reina had deigned to break the silence before getting too far to be heard.
“Let’s go to the Daikichi Mountain”, she said.
“Huh? Why?” Kumiko wondered, but followed obediently.
“I’ll push you down from it”, Reina explained.
They went up the stairs in awkward silence, cicadas chirping only emphasizing the lack of spoken words. Or was it only awkward for Kumiko? Reina was steadily leading the way, lighting the path with a flashlight, while Kumiko was stumblingly staggering behind not even daring to glance at Reina’s back, instead looking around as if in hope to stumble onto a suitable topic to start a conversation with, but there was only the darkness of the night forest. Well, darkness it is then…
“It’s so dark, isn’t it?” Kumiko noted totally failing to sound natural.
“Want to leave then?” Reina countered without even looking back at Kumiko.
“No, that’s not what I meant!” Kumiko hurried to assure.
“You know I’m going to invoke the promise, right?” Reina didn’t even bother to stop walking.
“I guess so”, Kumiko replied.
So that was it then. The promise they had that if Kumiko ever betrays Reina, Reina can kill her. Kumiko only wished she knew what her betrayal was. Not because she wanted to make sure it’s worth killing her. As long as it’s by Reina’s hands, Kumiko was fine to die on her whim. But Reina wasn’t a kind of person to kill on a whim. If she wants to kill Kumiko, she must really feel betrayed. And it bothered Kumiko that she had hurt Reina so much without even realizing it.
“Do you know why?” Reina continued her stiff-necked interrogation.
“Uh, well, in a way…” Kumiko mumbled uncertainly. “More like: is it that thing, I wonder?”
“That thing?” Reina has finally stopped and it didn’t seem like a good sign.
“Well…”
“If you don’t know, you should just say so”, Reina coldly decreed.
“But how am I supposed to know?!” Kumiko’s voice wavered as her clumsy pretense broke. “We were just talking normally! I didn’t do anything different! I didn’t say anything!”
“That’s the point”, Reina said.
“Eh?” Kumiko didn’t have the time to process Reina’s words, as Reina was already continuing her way up, so Kumiko followed shortly.

“We made it…” Kumiko sighed as they have finally arrived to the sightseeing platform. “So cold”, she shivered, as the forest didn’t protect her from the wind anymore.
Reina didn’t let herself be dragged into an empty conversation again and just silently came up to the railing. She gazed over the night city for some time and then…
“Aaaaaaaaaaaaaaaaaaaaahhhhhhhhh!” she screamed her lungs out as if trying to banish something from her soul.
“Reina…” Kumiko breathed out sympathetically, not daring to come by.
“Kumiko,” Reina called and Kumiko flinched at hearing her name. “You knew, didn’t you?”
“Eh?” Kumiko was still confused.
“About Taki-sensei’s wife”, Reina elaborated, the coldness in her voice giving way to grief.
“Ah…” Kumiko was startled now. “How do you..?”
“When we returned the keys to Taki-sensei, I saw the photo and couldn’t help but wonder. I asked Hashimoto-sensei and he said: aah, did Oumae-san tell you?”
“I see…” was all that Kumiko could say.
Finally, for the first time since they started their way up the mountain, Reina looked back at Kumiko, her eyes full of sorrow.
“Why did you hide it from me?” she asked.
“Because I didn’t want to hurt you”, Kumiko replied firmly.
“I know”, Reina said, her own voice wavering now. “You wanted the best for me and I can’t be mad at that. But even if you betrayed out of good intentions, I still have to kill you. That was the promise”, Kumiko noticed the teardrops forming in the corners of Reina’s eyes.
“I’m sorry”, Kumiko said. “So you weren’t ignoring me because you were mad”, she understood. “You were avoiding having to kill me”.
“I was shocked by how weak I am”, Reina said, turning back to the city again not to let Kumiko see her tears. “When I realized I’d have to kill you, I freaked out like crazy. *gulp* I… *sniff* I don’t know how I could even make a promise like that? I’m the worst…”
Kumiko came by and sat on the rail next to Reina, her back to the city.
“But I realize that at the moment I have to focus on the Nationals”, Reina dared to look at Kumiko again. “That’s why I called you here. To kill you off and forget about it for now”.
“Mm-hmm”, Kumiko nodded as she gently took Reina’s hand in hers and put it on her chest. “You might say once again that I’m awful for saying this, but his wife is no more”.
Reina’s eyes sparkled in the moonlight full with tears as she pushed the other girl over the railing.
“I’m rooting for you”, Kumiko smiled before slipping down.
A sad trumpet could be heard over the Daikichi mountain that night.

(Author’s note: here’s the Daikichi mountain sightseeing platform https://goo.gl/7Sw3KG It doesn't look like you can really fall do death here, but for artistic purposes let’s imagine there is a vertical cliff instead. It’s not like you knew it was a real place before I gave you the link anyway).