She decided to take things in her own way. The reason she didn't tell Karyk the way to break her seal was because she didn't want to scare him with this impossible task.
To break the seal, they needed the blood of hundreds of thousands of Purebloods. Just gathering so many of them was hard in itself, let alone without letting the Emperor find out.
Fortunately, Karyk solved that problem without even realizing. Not only that, he sent the Purebloods in the land of Chaos so they could train.
Instead of training them, the Puppet Master exhausted them with endless battles against the undead before slowly controlling the Purebloods, turning them into his puppets.
There were some who resisted, which ended up in a flight that he wanted to avoid. The Puppet Master won the fight against the inexperienced Purebloods, albeit not without getting hurt.