The key to understanding the scene is to recognize that, while Michael told Fredo later that "Roth wants me out," Michael is the one who wants Roth out. Roth and Vito Corleone had been partners, but only in the Prohibition-era booze business, as Roth pointedly reminds Michael several times. But Michael had been greedily horning in on Roth's Western gambling empire even before Vito died. The first business deal we see Michael involved in when he returned from Sicily was to push Moe Green, Roth's best friend, out of his Las Vegas hotel; later he had Moe killed because he resisted. By late 1958, when we first meet Hyman Roth, Michael has moved his entire operation to Lake Tahoe, owns three hotels in Nevada, and is about to force Meyer Klingman out of a fourth hotel, owned by Roth and the Lakeville Road Boys. And it's obvious that Michael has designs on Roth's Havana gaming empire. Small wonder Roth feels threatened!

Roth has been buying time by pretending he's a kindly elder statesman, in the twilight of his years, who regards Michael as his surrogate son and heir. But he's plotting all along to kill Michael—there isn’t room for both of them.
Roth decided to kill Michael at Anthony's First Communion party because he knew Frank Pentangeli would be there, contentious over the Rosato Brothers, whom Michael and Roth favored over Frankie. Thus Frankie had the perfect motivation to kill Michael--and wwould make the perfect fall-guy for the crime.
But the attempt failed. Michael recognize, almost immediately, that only Roth was subtle enough to have engineered the attempt while making it look like Frankie did it. It was a little too pat to be credible. Michael may finally have realized that Roth had more motivation to kill Michael (his entire gambling empire was at stake) than Frankie (whose only problem with Michael was the Rosato Brothers and their three territories). What would Frankie have gained by killing Michael--a freer hand in NY? But Roth had far more to gain--finally supplanting Michael as America's biggest gambling kinpin. How? Michael recognized right away that the attackers had help from "someone very close to us." That "someone" would be Roth's partner.