There is really only one situation where the believer is merciless and that is when dealing with the devil. He gets no mercy at all. Let me illustrate this for you.
My husband and I went to a prison to minister. As I was standing in the chaplain’s office I noticed a little bug crawling along the floor. I thought that was odd. How did this bug get in here in the first place? This is such a secure place, anybody that walks within the prison walls either works or serves here by invitation, voluntarily or involuntarily. I told the chaplain about this bug and without any further delay she got up from her chair and made a quick end to the bugs life; she took a stand and squashed him right there on the spot.
That’s exactly how we need to treat the devil: no mercy and no fair play. But please remember not to fight against a person that is under the influence of the evil one. You see you are fighting against an invisible enemy; these are rulers, authorities, powers of this dark world and spiritual forces of evil in the heavenly realms.
A good way to practice this is to charge your spirit by speaking out these verses mentioned in 2 Corinthians 10 over and over again until they are firmly anchored in you. That’s when faith enters your heart and your sword sways effectively against anything that is not of God.
Have a wonderfully blessed day!