As we know, after Chara died in their attempt to get 6 human SOULs with Asriel and break the barrier, Asgore found out and declared war on humanity. And unfortunately, 6 human children suffered this fate before Frisk fell. I'm going to give my viewpoint on how these 6 children died and had their SOULs taken.
Keep in mind this whole thread is just my opinion. You can disagree with me all you want, but just don't be stupid and start raising your pitchforks at me because I think differently than you. Also, I may refer to these humans with gender-specific pronouns. That's just me being headcanon-y with them, so one more time, don't be stupid.
One more thing I will point out is that certain characters will not be mentioned in this thread, as they've either never seen a human or are too nice. With that being said, Toriel, Papyrus, and Asgore are respectfully excluded from this, as Toriel believes in the goodness of children, Asgore never had to kill the humans on his own since they died before they could get to him, and Papyrus most likely hadn't seen a human himself until Frisk came, evident by his reaction to seeing Frisk in person.
So 6 human SOULs are shown to us when we fight Asgore. After we defeat him, Flowey barges in and foils our escape, crashing your game and deleting your save file. As Photoshop Flowey, he has the ability to call upon the 6 human SOULs and attack us with their power. He does so by using giant weapons, which you may notice to be the same weapons that are found in the underground. This means that the weapons and armor that were found in the game belonged to these children.
- The Toy Knife and Faded Ribbon belonged to Cyan
- The Bandanna and Tough Glove belonged to Orange
- The Ballet Shoes and Tutu belonged to Blue
- The Notebook and Cloudy Glasses belonged to Purple
- The Apron and Burnt Pan belonged to Green
- The Cowboy Hat and Empty Gun belonged to Yellow
Since these equipment sets are scattered across the place, it gives clues as to where they died. Cyan died in the Ruins, Orange died in Snowdin, Blue and Purple died in Waterfall, and Yellow and Green died in Hotland/Core.
So our first SOUL is Cyan. Since she died in the Ruins, she didn't make it too far. Toriel may not have been as overprotective as she was with Frisk, so she may have been a little laid-back and left earlier than she does in Frisk's run. This caused Cyan to wander around and eventually get attacked by a monster. The Cyan trait is Patience, which is the mechanic of taking damage unless you aren't moving. What this means is that Cyan may have done little to no movement in her battles, and if you have a brain, you'll know that it's pretty stupid to not move in a bullet hell game unless you're fighting Whimsun. In that case, she eventually died for being too patient in battle and not really avoiding attacks.
Our second SOUL is Orange, who died in Snowdin. Bravery's mechanic is pretty much the opposite of Patience, which of course means that you take damage unless you're moving. As such, Orange moved a lot in his battles, and there are certain monsters in Snowdin that use blue attacks that directly counter the Bravery mechanic, including Sans. Before you start yelling, keep this in mind. We don't know when Sans made his "Don't hurt humans" promise. It could very well be after a few of the humans that fell down died. With this in mind, Sans may have fought Orange and killed him, as his blue attacks counter movement.
The third SOUL is Blue, who is the first Waterfall death candidate. Judging by the placement of the tutu and ballet shoes, it's evident that she died early on in the area. If you remember, there's an early encounter with Undyne in which you hide behind a bush to avoid being seen. The Blue SOUL mode is a mode in which you jump to avoid attacks from Papyrus and Sans. Since this human had a blue SOUL, she was most likely jumping around in her ballet outfit, and what does that make you? An easy target. When Undyne saw this human hopping here and there instead of hiding in the bushes, she most likely speared her from hiding, ending her life.
Now for the fourth SOUL, Purple. Her notebook and glasses are owned by Gerson, which means that just like Blue, she also died in Waterfall. Since this particular human is perseverant, she pushed on despite the difficulties, which in turn got her far into the underground, but not far enough. Undyne says in her fight with Frisk: "I wanted this to be a fair fight. I thought if I beat you like this... It'd truly show how strong monsters can be." This quote signifies that earlier in her Royal Guard career, Undyne was much more brutal with her job, which means that she may have killed Purple very fast if she didn't give her a chance to do virtually anything.
Fifth, Green. First one to die in Hotland. We know the green SOUL trait is Kindness, which heals whoever is affected by it. In this case, Green was able to heal herself along with being kind to every monster she met. Now you may ask, how did she get past Undyne? We know that Undyne wanted a fair fight for Frisk, so in that case, going by what I said about Purple, Undyne may have felt a little bad with herself for being so unforgiving, so she decided to cut down on the brutality. She most likely did this for Green and Yellow, which gave them a chance to escape her spear. In that case, we know Green was kind with everyone, but there's one monster who requires you to be mean to him to spare him, and that monster is Astigmatism. You could say the same for Ice Cap, but Ice Cap merely soaks up the kindness while Astigmatism directly requests you to pick on him and chastises you for not doing so. In that case, Green's refusal to say anything mean may have caused the fight to last forever until she eventually died to the bully.
Last but not least, Yellow. This guy was a tough one to get rid of. He was an exemplifier of Justice and for that reason, he decided if monsters deserved to die or not. Seeing as how monster SOULS are made of love and compassion, Yellow spared them all, but there's one monster that actually has hate in them. Undyne. We know Undyne to constantly say things like: You know what would be more valuable to everyone? IF YOU WERE DEAD!!!" This mindset may have gotten Yellow to snap and start attacking her, which is the probable reason for the missing eye. Seeing this injury, the monsters may have gotten cruel with Yellow and just like Undyne was to Purple, didn't give him much of a chance. Since Yellow only had a gun to defend himself with, the gun ran out of bullets, since when Frisk finds the gun in Bratty and Catty's place, it's empty. In turn, the monsters eventually got the better of Yellow and killed him.
So there you have it. My thoughts on how the 6 humans that fell before Frisk died. Feel free to state how you think the humans died, but don't cause a flame war.