Derrim the mad is extremely paranoid.
He got two other derro killed when having a panic attack, screaming loud enough to attract the Neh-Thalggu.
He's mean and greedy.
He managed to find his way through the bore grass and set up camp safe from the other derro.
He's been poisoning the mushroom farm, using a secret corridor leading to the well, as revenge for having been tortured and thrown into the bore grass.
He'll shout at the adventurers if they try to advance without his consent which, may attract the attention of the Elder Neh-Thalggu.