I don't think that Michael thought that Frankie was behind the attack but he had to be sure and what better way to gauge that than to see how Frankie reacted to a surprise visit and Michael's display of real anger?

Michael had been frightened and angered and hadn't really had a chance to vent. Frankie was that chance.

As we know the "Michael Corleone says hello" was an adlib which was left in. It works because it not only is the sort of cold one liner which a movie bad guy would say while he's killing someone but it also adds to the audience's (and Frankie's) confusion about who's pulling the strings.

I think that Roth intended for Frankie to die that day. That he didn't and Roth was able to adapt was just testimony (pun intended) to how slick and devious Roth was. You don't get to be an old mobster without being able to think quickly in tough situations.