Enemies disappearing: Yeah it's that it's happening to humans +
Killing is a natural assumption when nothing is stated, but the characters don't seem like people who would just ignore killing a bunch of people, so it feels eerie.
Like going in, you're expecting that the slime society will be weird and alien in some sort of interesting way. When the slimes fought some humans and then decided to go try to trade with those people's "companions", I theorized (briefly) that slimes don't really view violence in the same way as people. That they can't really be killed with violence, so they think fighting isn't a big deal.
My perspective might be the weird one, though. It's been on my mind since I started thinking up games, that when humans (or creatures with human-like behavior) are enemies, I want to try and make them feel more human than they often do. That means giving them good enough AI that they don't look completely stupid, but it also means that violence has a certain gravity and says a lot about the person who does it.