John Gill Commentary Numbers 35:30

"Whoso killeth any person, the murderer shall be slain at the mouth of witnesses: but one witness shall not testify against any person that he die." — Numbers 35:30 (ASV)

Whoever kills any person
Willingly, and through enmity and malice:

the murderer shall be put to death by the mouth of two witnesses ;
which is repeated partly to show, that this law concerning the cities of refuge was not designed to screen a murderer, who was guilty through malice prepense; and partly for the sake of what is added to it, that two witnesses are required in such a case, where a man's life is at stake, to prove the fact against him; which shows how careful the Lord is, and men should be, of the lives of his creatures, that no man suffer wrongfully; which is repeated again and again, that it might be observed, see (Deuteronomy 17:6) (19:15)

But one witness shall not testify against any person, to cause him to die; which looks as if in other cases, in pecuniary matters, and the like, where life is not concerned, one witness may be sufficient; though it is always best and safest to have more if they can be had, that at the mouth of two or three witnesses everything may be established, (Deuteronomy 19:15) (Matthew 18:16) .