How do you say 'I really hate you' in Japanese?

本当に大嫌い。

hontou ni daikirai.

Improvement:

In direct speech, pronouns are omitted in Japanese, so the sentence above which literally means 'really hate' can mean 'I really hate you' but a literal translation of the sentence would be 'Watashi WA anata ga daikirai desu/da'. (Use desu if want it to sound formal, and da to sound casual. Using none of them is correct too)