I agree with TB to a great degree; but there is one more thing: the betrayal of omerta meant that Frankie Five-Angles was (from that point onwards) nothing in the mob world. He made enemies in the business, but was no longer a Don or protected. he would've died by any number of hands if he was on the outside or in a regular jail.

The Feds wouldn't have put him under such protection forever, now that he ruined their case for them he is no longer a "witness", Michael could've waited and killed him one way or the other when transferred to a regular jail.

BUT, If you recall Tom's visit to him, and the talk about Roman customs, it was to tell him that his family will be taken care of -something Michael didn't really need to do for a traitor, right?

Frankie committed a cardinal sin; he talked, for that he had to pay, but if he killed himself, his family will be taken care of - that's the way Michael shows that he "sympathizes" with him (as much a mafia boss can sympathize).... the other thing is sending a high-ranking member of the family to talk to him (Tom).... reminiscing on the old days, no threats, no "how could you?" .... but very friendly-like, that means RESPECT.

Finally, suicide does not send a message regarding Michael's retribution towards traitors, it does not "Make an Example"..... that's what a brutal bloody death does....