My impression is that "killing your darlings" or, rather, editing down / revising, doesn't usually happen on as drastic of a scale as is described in the OP? I'm sure some authors work in that way, where they scrap so much that the project barely resembles itself by the end, but I think in most cases it's more subtle 
It's more so about editing down, getting rid of extraneous aspects or elements, and trying to tell the story in a concise and clear way, I think. The reason that it's "killing your darlings" is because often a lot of the "fluff" that comes up in the drafting stages of a story are elements that we think are really cool- like a character we really like the design of, or a particular character interaction, or maybe several side-McGuffins in addition to the story's main McGuffin, or what have you. Sometimes these fit in a story well, but other times you have to go out of your way to reach them, introducing the aforementioned fluff, or otherwise they might just not be critical to telling the primary story and it's possible that by cutting or combining some of those aspects you can arrive at a tighter, better story.
I'll briefly talk about this process from my first Tapas one-shot comic. The original premise was that 2 teams of treasure hunters (each comprising of 4 members) were going to compete with one another to retrieve their town's important treasure that was stolen overnight.
Paragraph Summary of Rising Action
One team was originally tasked with the mission but the other overheard and wanted to thwart them and steal their job, basically. This resulted in a night time showdown between the teams as team A was settling in to camp for the night, where team B ambushed them. At the conclusion of the fight, team B releases some sleep gas on team A and continues on to pursue the thief. When team A wakes up in the morning they rush to catch up, only to find team B imprisoned in a stone cage. After freeing them team B informs A that the thief did that and shares the intel that they were able to gather from their encounter. One member from each team, a fire and water mage, then team up to go pursue the thief while the other 6 hold down the fort outside her base of operations. Then the climactic battle happened.
The short of it is that I ended up cutting most of the above content lol. My original project goal was 50ish pages and I was just trying to cram way way too much stuff into a short story. Most of the characters would have seriously lacked development and it would have ballooned the page count way beyond my initial scope (or had to have been rushed), neither of which I wanted for my project.
So, I ended up cutting out 4 characters total, the camping/fight scene, and the whole "rival team" subplot, and revised the rest of the events to work with the new character cast. Overall the final product still pretty closely resembles the original outline, but it was edited down to remove fluff and tell the story more clearly.
Like, the 4 characters I cut I was excited to draw and I thought they had fun designs and powers but... aside from the side plot (in an already short story), none of them really served a purpose and would have bogged things down in the big picture. Retrospectively, I should have actually cut a 5th character too- one of the 4 that I left in basically does nothing and could have been removed... but oh well~