The reason why Zhou Tian first targeted the ogres was simply because they were larger and easier to kill. However, when Zhou Tian realized that just showcasing a part of his power was enough to scare the goblins, he knew that his previous thoughts were not comprehensive.
Given the nature of the goblins, perhaps Zhou Tian did not need to defeat them at all, but merely demonstrate his power, making the goblins feel they couldn't compete. He reckoned that even with fear, he would have the ability to scare all the goblins away.
Once such a thought emerged in Zhou Tian's mind, he immediately took action.
Attacking the ogres had already affected the goblins. What if, after killing some ogres, he suddenly turned around and attacked the goblins?
Zhou Tian wanted to know whether the goblins would have the courage to resist when faced with his attack. After he had killed a group of goblins, he wondered what the other goblins would do.