My take: To set the tone. To help use feel the despair and insanity the characters are feeling.
To show how fucked up everything is. How hopeless/wrong the situation of the main cast is.
To make u feel the utter insanity of the situation.
And how, to save humanity, they chose to let their humanity go.
To show low they are willing to go to free themselves of Almuten, and grant themselves some reprieve.
Thankfully, I don't think rape here was ever shown as a "forgivable".
Oh God. I'm sorry X< i kinda wrote that w/o thinking it thru.
Anyway, in shorter words, opening it with a rape makes u hate them and the situation immediately and makes u less sympathetic w/the scions of humanity.
Also, it's can b a bit intriguing (at least for me....... like what pushed them to go that far)
I totally get what you say like in other psychological or shounen mangas they use killing, torture(or sometimes even rape) as their tone setter however in bls usually they connect emotionally with their villain(cuz theyre usually ml for some reason) and for me it just destroys the whole thing. I hope this doesnt turn into this.
Oooohhh, I guess this just ain't for u then. (Too bad, but, I respect ur boundaries & known its matter of personal taste)
Yugeon does end up w/Shinje (blondie leader), but to my understanding never actually fully forgives any of them for what they did. (Based on the spoilers I read)
Now, as for why, I have an inkling. But I can't say for sure yet since I didn't read the novel and this manwha ain't finished yet.
Look i didnt continue to this because it has rape but when i look at the last chapters it looked like action bl rater than rape so my question is if u had a plot from the begining why the fuck did you add rape why? If the story is about gates and espers why would you add rape to the start of the manga i dont get it. Dead man switch (it is action bl as well) isnt like this so why? Just please dont end this manga romantic.