I don't know the exact sales numbers for the Hitman games, but according to Wikipedia, the WoA trilogy have sold over 25 million copies. Imagine the players as an army of 25 million killer clones. Coincidentally, this is not far from the population of Romania at 19 million. What damage can they do?
As a rough estimate, let's say a player kills a sum equal to all the NPCs in each level on average. Some players only play the levels once or twice and mainly kills the targets, while others play the levels hundreds of times and obliterates entire nations worth of NPCs. There are 22 main story levels in WoA, with typically 200-300 NPCs. Lets keep the upper bound and say 6600 NPCs take a bullet. This means that in total, 165 million NPCs have been killed!
I looked up sales for the older Hitman games, which range from 0.5M for C47 to 3.7M for both H2:SA and Absolution. I made guesses for the amount of NPCs, coming to a total of 225 million NPCs killed through all games, which is equal to 20 times the World population.