That's not true if I recall correctly the book explicitly that it was easy for him and his men at first because they were just killing the turned undead.
Then it got really hard because they were killing people who were not even trying to defend themselves because they saw the soldiers as there to protect them. Finally it got a bit easier again when the townspeople took up arms to try and defend themselves.
Pg 104. "The ones that had risen were easy. They were the enemy; human no longer, but vile caricatures of what they had once been in life, and smashing their skulls or slicing their heads off was no more of a hardship than putting down a rabid beast. The others—"