Just to keep track of the message clues left by the murderer with each corpse. So far, they are all from the Analects of Confucius.  As translated by WeTV.

1.  My principal is unified.
吾道一以贯之

2.  Six or seven young ones
童子六七人
Full passage: 莫春者，春服既成，冠者五六人、童子六七人，浴乎沂，风乎舞雩，咏而归
Clad in attire in late spring,
Grown and young ones
Bath in the Yi river
And the wind on the altar
Homeward with songs

3. Standing by the river, the Master said "Time passes like this."
子在川上曰  逝者如斯夫

4. Such a person could only get such a disease
斯人也而有斯疾也
In contect of the case a doctor could be killed by his own medicine.

5. In youth his vital force was unstable he should guard against lust
少之时而·气未定 戒之在色

6. Worship the wrong ghost.
非其鬼而祭之