So. In relation to Temmie's "situation". As far as I'm aware, the only reason the "Soulless Ones" aren't evil in Fell universes is because of the fact that, since the monsters are more violent, the Flowey/Equivalent doesn't have the opportunity to start killing. However, that explanation seems a bit... hollow. (No pun intended.) So, here's the lore for the KK!FellSwap take:

When Boss Monsters die, their SOUL lingers, then shatters. However, when normal monsters die, their SOUL turns to dust along with their body. So, in Undertale, when Asriel dies, his dust, and therefore his essence, spread on a flower, but his SOUL shattered into pieces. So, when Flowey was injected with DETERMINATION, Asriel's essence was restored in the flower body. However, when Monster Kid, who is NOT a Boss Monster, died, the dust of both their body AND their SOUL spread on the plush toy. So, when the plush toy was injected with DETERMINATION, MK's essence was restored in the plush toy body, along with their SOUL, therefore resulting in Temmie having a SOUL, therefore giving them the ability to feel empathy, love, ETC.

If you read through this wall of text, good job!

Welp, until next time, have a good one!

-KK