... I believe that is a proper standard by which to judge people, ... (Para. 7)
Meaning: I believe that is a proper standard by which I can judge people.
Notice that in this sentence "by which to judge people" is a relative clause in which the finite verb structure is replaced by an infinitive verb. This is a very common structure in English:
I don't know which way to go (= which way we should go) to reach the nearest branch of the City Bank. 我不知道走哪條路才能找到離這里最近的城市銀行分行。
He was not able to explain how to go on (= how we should go on) with the experiment. 他說不清該如何繼續進行這項實驗。