The issue wasn't so much that Faith accidentally killed a man, it was that she DIDN'T CARE. That's where the relationship fell apart between the two slayers.
One death is a tragedy and a million is a statistic -- Stalin
My grandfather was in World War II as a American serving in the Pacific Theater. He did see combat in the Philippines, when it was still a American territory since the Spanish American war and gained independence right after World War II. True, if you are born within the United States, you are a American citizen. But, even that the Philippines was American territory making it within the United States -- being born there you were not declared a American citizen because it was always believed that the Philippines would become a independent country. Second, Americans do not leave the United States and declare themselves independent. During the occupation of the Philippines by Japan, the death toll was 1.1 million people. When you read about America during World War II, you are told about how many Americans were killed during the war, and that number us under 500,000. The better question to ask, how many people were killed within the United States during World War II, and that number would be over 1.5 million people. With just this one factor, and with others -- North Vietnam understood that the Americans could not win the Vietnam war because the Americans care more about Americans with American citizenship then Asians born within a American territory without citizenship as a American.
Now, more Americans care more about Ann Frank being a young teenage girl locked in a attic with her family and other people then the 1.1 million people killed in the Philippines. They care more about Ann Frank having curiosity about her body changing when she asked her older sister if it was fine to compare their breasts. That is more interesting to debate within the American intelligencia then the vast amount of people that was born under the American flag as a Asian without citizenship who during the war fought and died to defend the United States.
You say, how does this lead up to Faith not caring about killing a man. As a American, I live in a country that does not care about Asians who grew up under the American flag, given a education to be like Americans, was given a education to read and speak English then Spanish. Then, they had to deal with Japan, and was being killed by them and had to defend themselves. My personal education has been to be rational. If I can be rational about the treatment of the Philippines during World War II, then I can support the feelings of Faith killing a man. Faith is a fictional character that killed a fictional character and was acting out as being rational. Yes, she did not care about killing him. But, it was cold and heartless, but it was a rational outcome. Just like my fellow Americans that look at the 1.1 million people that was killed in the Philippines during World War II. Yes it is cold and heartless too, but, even that they were born within the United States -- they were not American citizens.