Is this the worst line of dialogue in the entire history of superhero movies?

I don't know if you saw the theatrical cut *only* or extended edition, but there's no reason to settle for the theatrical. It was a bad movie that removed so much of the essential narrative as to render it almost non-sensical. It was likely done to fit more showtimes in during the course of the day in an effort to pass the $1 billion box office goal.

The extended is the ACTUAL movie where the events are clear. In that movie, Lex Luthor is not just a token bad guy who is after kryptonite, Zod's body, and technology from Krypton. He was much more than the creator of the Doomsday monster.

Batman v. Superman Extended has two equal protagonists in Batman (Wayne) & Superman (Kent), and one main machiavellian antagonist in Lex Luthor. No matter what other flashy comic-book spectacle the film has to offer, the crux of the story is simply how Lex Luthor has a scheme to define what Superman is in Wayne's mind, and simultaneously who Batman is in Kent's mind.

So, when Bruce Wayne and Alfred have this "1% chance" conversation, it's almost like Wayne is indoctrinated. He has been carefully manipulated by Luthor to behave unlike a traditional Justice League superhero that you seem to expect. Instead, it's the typical comic-book-Luthor's rationalization for the destruction of Superman being spoken by Wayne as if Superman were Batman's archnemesis.

It's only when the two superheroes have an actual conversation post-battle (The famous "It's his mother's name" scene) where Wayne sees the human side of Kent for the first time. Despite being a little clunky in it's presentation, I have no idea why that scene receives so much criticism—particularly from those who could follow the details of the extended version. It has nothing to do with Martha or the name itself, and everything to do with the fact that Batman finally sees Superman for the first time using actual logic. That logic is that there is a nil% chance that a child raised in Kansas by heartland farmers who actually cares for those people could be an enemy of Earth or a threat to humanity.

The intricacy and execution of Luthor's scheme to get them to battle is where the heart of the film's greatness lies. If any footage of his plot is cut out (which it was) then the viewers are left wondering why Batman is not Batman and Superman is not Superman. That's what you have done here with your OP.

Batman is incredibly calculated and thorough. Read the Dark Knight Returns if you don't know what I mean

You're right, the probability is much higher.

Look at all the Kryptonians that reach Earth, most of them are evil assholes. Even humans are more than 1% assholes and given the chance and unlimited power they would fuck up everything in a heart beat for their own gain.

Putin has a 90% support rate in Russia ffs.