[LROlsson]

Originally Posted by Xellos-_^

are the Red Slime still sentient or are they acting on instinct?

They have some sentience. When you fight them, they say stuff like: Stop it, don't leave me, it hurts, Mama.

As for the red slime in the beginning of the story

Spoiler:

In the game, I got the impression that it could be Haku's niece, and in the new scene in the anime it seems to hint at it even further with the slime looking at Haku, and then leaving.
Too bad it's impossible to know whatever it is her or not until it might be revealed in Utaware 3.