Varis was shown to care deeply for Regula. He also seemed to show some hesitation when pressed about Zenos' demise. He may have written him off as a monster but the man was still his son and heir. It's possible he did not wish to put down his own son, or simply sent him away on purpose to be of use far away from the throne itself. Or, perhaps, he was sent their deliberately knowing that there was a possibility that he would not return home. Varis didn't send any reinforcements, after all.