Because there's a difference between Chaos Dwarves and chaos dwarves.
These might be the only one on this side of the planet that we could have a chance to speak openly with.
Doubly so with all the Rune Lords stuff.
I'm pondering if this beastman is similar to K8P's golem, with runes inscribed on its soul. I wouldn't be surprised if there's a Dwarf, living or dead, that's 'piloting' it
OK, why do we want to speak to servant of the Ruinous Powers? Why and ow should we trust them not to betray us?