Welcome, Krlea!
Tom may have regretted his role in Fredo's pending death. When Michael returned from Cuba, the first thing he asked Tom was, "Where's my brother?" He then told Tom to tell Fredo, "Tell him I know Roth mislead him..." etc. But Tom was smart enough to know, then and later, that Michael never, ever gave a pass to an enemy or to someone who tried to harm him. Tom must have known that he was being used to lure Fredo back to Michael so that Michael could get more info out of him. Tom also must have known that Fredo was a dead man after that. So, that regretful look may have been Tom's acknowledgment of his own role.