If Fredo could be so easily suckered by Roth's goons, he could be suckered by anyone, including law enforcement. That's the way Michael saw it. Fredo is a made man. He took an oath, and his actions were treacherous. No coming back from that. Sure, he came up in a mafia family and likely didn't have much of a choice, but in his case, it turned out to be a curse. He wasn't cut out for the life. Should have just been on the legitimate business side without having any knowledge of the rackets. Could have still been a Vegas playboy working for Benny Siegel (Moe Greene), which was probably his calling.