The average man is easy to read for an astute woman, particularly one who's touching hands across a table and starring into his eyes, while confusing him by looking and smelling gorgeous. Paolo was no more than that average man. He was involuntarily giving himself away.
The mere mention of Livia... and the suggestion that she was implicated at best, and responsible at worst, for her husband's death and attempts at extortion caused a Newtonian response. He raced to defend her, then realised that in so doing, he had inadvertently acknowledged that to do so she must have had help.