The risks Cady was taking (possible spoilers).
Possible Spoilers!
Throughout the movie, it's repeatedly said that Cady is being very clever and cautious in his acts. Obviously he doesn't want to go back to prison (as is made clear by his reaction at the end, when Peck tells him he's going away for life).
Yet so much of what he does carries huge chances of having just that fate befall him.
Assaulting Diane (the bar girl) carries the risk that she would press charges and testify against him. Attacking Peggy in the houseboat is even more likely to result in his being arrested, tried and convicted. And, of course, his final attack on Nancy also holds great risk that she would indeed testify against him. Even the story he tells Bowden at the bar, about how he went after his ex-wife after he got out, was an enormously risky thing to do.
Cady was gambling in each case that none of the women would testify against him out of fear or embarrassment. Maybe, but each of these assaults was a completely stupid thing for him to do, especially as they could have stopped him from carrying out his "primary mission", getting revenge on Sam Bowden.
The letter he said he had his ex write could have been easily challenged in court as written under duress (which it was), and not as an alibi for his actions (which surely no one would believe the ex-wife would have agreed to). There was no guarantee Diane wouldn't go after him, and there was evidence against him there. Peggy would absolutely have pressed charges and testified against him. Even Nancy might have been willing to come forward and undergo the ordeal of a trial just to put Cady away.
Point is, for a supposedly careful man, Cady engaged in some very violent behavior with absolutely no guarantee that the women involved would simply (and conveniently) let the assaults go. All this is at odds with the allegedly cautious and systematic behavior he was supposedly adopting in order to get to the Bowdens.
Not to mention (as I did in another thread I just posted) that all this ignores the fact that he murdered the policeman assigned to guard the Bowden family at the houseboat. This alone would virtually guarantee a death sentence for him. That was his stupidest action of all.
Did Cady actually believe he could evade prison after all these blatant crimes? This is the one big flaw in the film's story.