We (and a lot of the Asgard gods) tend to think of giants as stupid, but they actually come in all shapes and (brain) sizes. Geirrod's brain is one of the bigger ones. He's not at all fooled by Loki's bird disguise, and he's pretty quick to figure out how he can use his catch to get the person he's really after: Thor. Even his plan to kill Thor by enticing him to play a "game" of "catch the molten iron" is kind of clever, since it takes advantage of Thor's natural competitiveness.
Nah, Geirrod's no dummy. As Grid describes him, he's a tricky son-of-a-gun. But he also has a cold, cold heart. When his daughters are gruesomely crushed under Thor's chair, he doesn't even bat an eye. That makes it hard to feel sorry for him when Thor returns his molten iron right through his stomach.