Nice observation: “If the JWs had political power, they would kill also.” In their thinking they treat the ex-witnesses as those who have already being killed.
Now, regarding your OT apprehension:
All those God-authorized killings are attributed to God so that genocides assume validity. Jesus says many things in the OT are not original. (Mathew 19:4-8) In the great fire of Alexandrian Library, all the original scrolls of OT too were destroyed. What we have is the copy of the copy of copy ….. So is the case with NT scrolls. We need to take only what is REASONABLE. (1 Thess 5:21, 22) Mathew 5:44-48 and James 2:8 are universal in application, and its validity can never be questioned or doubted because it does not distinquish between JWs and gentiles/apostates!
Even many things that are supposedly said by Jesus is actually attributed to him. For example, the lengthy last super speech has never taken place, in which the new commandment is introduced. See, Apostle James is not at all aware of such a new command or speech. Because, he invokes “Scriptures” (not Jesus Christ) as authority for saying love your neighbor as yourself:
“If you really keep the royal law found in Scripture, “Love your neighbor as yourself,” you are doing right (James 2:8)
Jesus would never make thoughtless, God-dishonoring statements. Yet we find him doing so in John 9:1-3 where he presents God as creating people with physical deformities for "HIS GLORY"—God as a sadist—another example of saying attributed to him!