(お前は私に)「～～ volitional auxiliary う/よう」と言った。
(お前は私に)「～～ plain form of a verb」と言った。
I would normally interpret the former as "You said, 'Let's do ~~'," i.e. "You suggested that we should do ~~," and the latter as "You said, 'I will do ~~'," i.e. "You said that you would do ~~."
So I think 「お前は『人間になる』と言った」 means "You said you would be human," and 「お前は『人間になろう』と言った」 would be interpreted as "You suggested that we should become human."
- 彼は(私に)東京に行こうと言った。 He suggested we go to Tokyo.
- 彼は(私に)東京に行くと言った。 He told me that he was going to Tokyo.