Well, this is an interesting nuance:
When Michael said he was "surprised," I think he was being sarcastic. He could have been referring to the fact that Tom was an educated man and should have learned something of world history. Or he could have been referring to the many murders Tom was involved in--including McCluskey's, when Tom had said that the Corleones would "become outcasts" if they whacked a police captain, who was (according to him) "invulnerable." Nasty business, emphasized by the smirk on Neri's face when Michael tells Tom that he can "take your wife, your family and your mistress, and live in Vegas."