It takes a whole lot for a troll to be considered so exceedingly stupid, dangerous and unpredictable that it stands out by Troll-standards, but Glistrup is an old troll and his eyesight and sense of smell isn't as keen as it once was, and mistaking a chaos dwarf Slavemaster for a goblin and attempting to eat him was a perfect way to get a new work assignment. It isn't really clear though, why Glistrup wasn't put to use as a shooting target on the hellcannon range, but a reasnoable guess is, that the powers that be, place little faith in Blood Bowl Theraphy curing anyone, and hopes that Glistrup will eat up every single chaos dwarf deviant on the team.