Halakhah su Numeri 35:30

That the witness not issue a ruling in the case that he is testifying about in capital punishments: That the witness not speak about the case that he is testifying about in capital punishments, except for his saying his testimony alone - and even though he is educated and wise; as the witness is not made into a judge in capital punishments, as it is stated (Numbers 35:30), "and a single witness should not respond about a soul for death." And Rambam, may his memory be blessed, wrote (Sefer HaMitzvot LaRambam, Mitzvot Lo Taase 291), "And the negative commandment about this matter is repeated, as it is stated (Deuteronomy 17:6), 'he shall not be killed by the mouth of one witness' - meaning to say, he shall not be killed by the ruling of the witness. And they said in Sanhedrin 33b-34a, '"And a [...] witness should not respond about a soul," whether for innocence or whether for guilt.' And they explained that the reason for this is that it appears as if he is biased in his testimony. And this matter that he cannot respond - whether for innocence or whether for guilt - is only with capital punishments."