Answer: deceitful; untrustworthy
Step-by-step explanation:
The term perfidious most clearly mean as used in paragraph 6 in Of Revenge means to be deceitful or untrustworthy.
A perfidious person is someone that is dishonest, lies a lot and untrustworthy.
For example, the sentence "John is a perfidious" simply means that John is not someone to be trusted.