I was watching this anime events clip -> https://youtu.be/5y15wDpcwuI?t=6m12s
and could not understand this particular part.
at 6:12 I think the seiyuu said
そうなろうと思えば、 この世界はどこまでも残酷になれるのだということ
I could not find this word in the online dictionary, so maybe I heard something wrong or is it possible that it simply mean "that".
At 6:23 I could not understand clearly what the other seiyuu said?
I believe it is
遠い昔に書かれたあるXXX
XXX節がある
そうなろう is composed of:
そうなろうと思う literally means "(someone) thinks they want to become so".
And this そう refers to something said after it — 残酷になる.
そうなろうと思えば、この世界はどこまでも残酷になれる
If this world wishes (to be cruel), this world can be infinitely cruel.
Yes this is an example of personification.
it simply mean "that"
No. そうなろうと思えば means "if (it) imagines(/wishes) (it) could be."
遠い昔に書かれたあるXXX
It's very hard to hear the word, but it seems she is saying「絵本に」.
XXX節がある
こんな１節がある
そうなろう is そう + なる.
The verb なる means "to become" and it is in "volitional" form here. It's a little hard to translate literally, I think, but "そうなろうと思えば" here is like "If you think things might get like that..."
As for the second part, I can't say with 100% certainty that I'm right here, but it sounds to me like
"遠い昔に書かれた絵本こんな一節（/一説）がある"
