Assuming they are two actual beings, I think they'll be angels who have taken on the appearance of men.
Human beings would not cope with the sort of power these two "witnesses" will have, and it would go to their heads. Whereas for angels they would just be doing their job as instructed. They also would not have humanitarian feelings like we do.
If the Angel of Death that passed over the Israelites in Egypt, but slew the first born of all the Egyptians had been humanitarian, there would have been complaints forms in triplicate lodged with the Angelic Rights and Ethics Committee.
"I'm not doing that!"
"Yes you are!!" said God. "No I'm not" said the angel.
He was given a job to do and did it, no questions asked. I think it will be similar with the two witnesses, assuming of course they really are two individual witnesses and not Revelation-style symbolic language for something else.