Jesus was instructing his disciples in the time just before he was arrested. Warning them of troubles they would face, He said, "a time is coming when anyone who kills you will think he is offering a service to God" (Jn 16:2).
Who are the actors in this statement? If you accept that Jesus is an authority on the identity of God, then this relates to the one real God. Jesus was speaking to his disciples the very first Christians. And he was talking about killers that would not be acting as agents for God.
The Bible contains support for capital punishment. The civil law of the Old Testament Israelite people included capital punishment for murder (Num 35:16). This body of law was part of the contract that God made with His people. The law was good stuff and helps us understand God's moral nature. It also showed us that punishment is the responsibility of due authorities and not vigilantes (vs15). However, this contract has probably expired. Certainly it is not binding on Gentile residents outside of Israel.
There is an older contract. Before the flood, humans were only given plants to eat. But after the flood, God made a covenant with Noah (Gen 9:1-17). He promised never again to destroy all life in a flood. He gave permission for humans to be meat eaters (vs3). And He required capital punishment for murder. All aspects of this contract still apply to everybody.
But nowhere does God require a life because of a person's beliefs. God will judge us for what we have done (Rev 20:12). It is true that our actions will flow out of our beliefs. A Christian's primary task is to love. First love God (Mt 22:37-39), and second love other Christians (John 15:12&13). This leaves no room for killing.
Jesus repeatedly had to correct the people around Him as to what kind of a kingdom He was starting. His was a spiritual kingdom and not an earthly kingdom. Jesus explicitly pointed this out to Pilate at His trial (Jn 18:36). God's servants don't fight with human weapons to advance God's kingdom. In fact, the killers of heretics that Jesus warned about don't even know God (Jn 16:3). And so we can see that any religiously motivated killers do not work for God.
This puts a clarifying filter on events of today and events of church history.