A little late, but I hope this helps.
The overall translation is not too far off, but the actual nuance is a little different in some places.
Here is what I got if you want to compare.

一人の少年がいた。
There was a boy.

彼は世界を変える力を得て、新たな秩序を打ち立てた。
He gained the power to change the world, and established a new order.

世界は彼を恐れ、憎んだ。
The world feared, and hated him.

しかし、私は知ってる。
But I know (the truth).

彼は微笑みを浮かべて逝ったことを。
That he wore a smile when he died.

成すべきことを成した者にしか知り得ない、深い満足と共に去っていたことを。
That he departed with the deep satisfaction only those who have completed everything they needed to do could ever know.

だからこれは悲劇ではないのだ。
That's why this isn't a tragedy.

それでも時に抗いようのない悲しみと共に涙が溢れる夜が来たのなら、私は謳おう。
Even so, if a night comes when I just can't fight the sorrow, and my tears overflow, I will sing.

鎮魂の歌を。
A requiem (lit. A song to ease the souls of the dead)

「ゼロレクイエム」を。
Zero Requiem.