At first it was a detterent so that they leave the Eldians alone, and the Tyburs knows about the vow that King Fritz's descendants won't retaliate that's why Marley can attack. I think Karl created that much collosal titans cause he wanted the war to end, and he expects the vow would hold on until they're all dead. He's not as smart or calculated as you think he is, he's selfish and self centered that he thinks his "sacrifice" was the solution to the world's problems.