There are many answers covering this piece of grammar called `rhetorical negative`, like this one I asked myself over 3 years ago...
Basically adding or / to the end of a sentence can turn it into a rhetorical negative.
exploring your example:
> - Mario-san! That's cruel. / You are cruel.
> - Mario-san! That's not cruel. You are not cruel.
> - Mario-san! Aren't you cruel?
If we want to be more specific, and perhaps have a more natural translation:
> Mario! Don’t you think that’s cruel of you? (Leaving me there) or Mario! You are so cruel, aren’t you?
In the polite form, when using this rhetorical negative you will definitely end it with ``. And for proper affirmations you will end it with ``.
It gets trickier when we're using informal language because both affirmations and rhetorical negatives can end with just ``.
In that case you will only be able to tell the difference by the speaker's intonation, and / or by the context.