Tom was involved in any number of murders, so the reference wasn't to that. That Tom was so knowlegeable about history is what I think underlies Michael's "surprise" reference.
Refresh my memory here olivant. Where is there any indications made in the GF movies that tells or shows us that Tom is SO knowledgable about history?
And if anything , it would be BECAUSE Tom was involved in the planning of so many murders that Michael would be surrised that he, Tom, would make the statement that he makes about Roth.