Compare this to "The Godfather". At the beginning of the story Michael is this decent guy who fought for his country when he could have gotten out of it, doesn't like what his father does but still loves him anyway and is very good to whatever Diane Keaton was at the beginning of the film (wife, fiancee, girlfriend I forget). He's not exactly Mister Sunshine but he seems like a decent man with a set of morals. So you kind of like the guy that he is this way despite the environment he grew up in.
Then he kills the guys in the restaurant, but they were corrupt and trying to kill his father anyway, so you can justify it. But then, because Sonny gets blown away and Fredo is an idiot, he's suddenly thrust into the position, he never imagined he'd be in, where he has to take over when his father dies.
At first you think he's going to be different than his father, but no. By the end of the film he's having his enemies killed in cold blood at the same time he's renouncing evil, then he let's his brother in law believe he's going to be spared before having him killed and he lies to his own sister about doing it.
Then of course in GF 2 he commits what many would consider the ultimate act of evil when he has his own brother, who he knew was kind of a helpless doof, killed. Not only that but he lets Fredo think he's forgiven him at the exact same moment when Michael is giving the silent order to have him killed. So it turns out he even more ruthless than Vito ever was.
If Michael had been a complete jerkoff from the start this might not have been so surprising. But the fact he did seem like a decent and likable guy at the beginning of the series made it absolutely horrifying when he does the things he does.
Whether is was lousy acting, directing, or both the fact Valeris comes off as so unlikable from the start makes it totally "Oh, who cares, she was a bitch anyway" moment when the big "surprise" as her being a conspirator is revealed.