So I have a question regarding corruption points:
In a recent session, an NPC disguised in rags had hidden himself fairly well along our path. When our group approached, he jumped to his feet and shouted about a darkness within our group (pointing to the pysker). The psyker in the group got an odd sense from the man (he gave no presence in the warp), so the sentiment was similar from him.
However, the man quickly went from talking about darkness to saying we were Inquisitorial agents. He also knew we needed to find a way out (we were being chased), and knew the exact location of the exit. In a brief dialogue, the man said several questionable things which, to my belief, were heretical. First, he made specific statements with regard to the Inquisition being a path leading to darkness. He cited being a former agent of the Inquisition and a witch/psyker hunter and having found himself falling.
My character, being a Cleric and having a background (from Inquisitors Handbook) of Redemptionist, said "We are servents of His Holy Inquisition spreading His Righteous Light." To which the man responded "The brightest light casts the greatest shadows." To me, the entire line of conversation and in particular the line at the end, in addition to the shadiness of knowing things led me to think: heresy/heretic!
So as I passed the man, I drew my shotgun and screamed at him "Burn Heretic!" at which point I fired an inferno shell and he burned. As an additional note, the man was drawing his own weapon -at the same time- as I was drawing mine, and the GM ruled that we both fired at the same time. Firefight ensued, he was slain, with the psyker turning to fire at him as well.
My concern is this: following the session, the GM asked myself and the pysker member of the party to make Willpower tests. The psyker failed, and I succeeded. The GM then revealed that each of us gained a corruption point. When asked why, the GM said it was due to the man we had killed. His justification was that the man was innocent (had 0 corruption points) but was just incredibly insane (70+ insanity or so). Thus, the GM ruled that our killing the man, who was innocent, was a corrupting act. He added that my Cleric was in danger of up to 1d10 corruption, but since I passed my Willpower test I only took 1 point.
Is this a generally accepted mode of thought? Or is corruption something that most people feel is reserved for more severe things (warp influence, forbidden lores, etc)?
Thoughts appreciated!

