Of course Shmoop doesn’t blame the baby. He didn’t ask to be conceived. He didn’t intentionally kill Catherine when he was trying to be born. But, as we discuss in the baby’s "Character Analysis," Frederic blames him for Catherine’s death.
Other than the war itself, unless you count the baby, nobody is really out to hurt the heroes or make their lives harder. And the war is all the antagonist we need, isn’t it?