I haven't read Empire so I can't say for certain, however if those packs are anything like the ones watching Sanguinius in Fear to Tread then they're not really executioners. There's never any suggestion, and it would be rediculous of there had been, that these Wolves would be capable of executing a wayward Primarch. They are simply there to watch and report back. When a unit is actually sent to provide potential punishment on a Primarch the Emperor sends Custodes not Wolves.
"Look into my eyes, and see your death."
"Let them hate, so long as they fear."