After the Scorched Earth Rat King took notice of Hugh, it discarded its previously leisurely pace as it started running. Compared to the stationary town where a lot of humans would be even if the King didn't rush over immediately, the lone human could easily escape and get out of the King's reach if it didn't hurry.
While thousands of low level humans and beasts were attractive, there was nothing better for the Rat King to improve its strength than higher level food. Although Hugh was only a fifth level mage, the Rat King had so far only consumed mana from low level creatures. With the qualitative difference between low and mid level, Hugh alone would help the Rat King's strength grow explosively.
Although there was no limit to how strong the Rat King could grow even with low level nourishment, after some time, the efficiency would be lacking, and to actually grow significantly stronger, the Rat King would need higher level food.
