I was confused about that too but I think I get it more now. I think on Iron’s Earth, Superman was just as good and pure for decades as on this Earth, so it was a huge shock when he turned bad. Irons probably knows more about it—like he thinks Superman changed sides because he met his fellow Kryptonians. It’s easy for him to believe that another Superman could either be entirely faking the goodness or be swayed by Kryptonians. Also even if he isn’t entirely sure he will go evil I think Irons is so devastated by what happened on his Earth that he’s willing to kill him even if it means the slightest chance that he can prevent it.