YOU SAID:

knew he was a killer first time that I saw him

INTO JAPANESE

初めて彼を見たとき、彼が殺人者だと分かった

BACK INTO ENGLISH

The first time I saw him, I knew he was a murderer.

INTO JAPANESE

初めて彼を見たとき、彼が殺人者であることがわかりました。

BACK INTO ENGLISH

The first time I saw him, I knew he was a murderer.