Albert Barnes 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)

By the mouth of witnesses - that is, two witnesses, at least (compare the marginal references). The provisions of this and the following verses protect the enactments of this chapter from abuse. The cities of refuge were not intended to exempt a criminal from deserved punishment.