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Killing with a knife is way harder because it's not like you can practice before hands. It's a messy and butal to do and the very thought of it makes me cringe. Guns on the other hand seem to kill people much more cleanly(e.g. shot to the head).

Killing with a gun is even easier for the people that are avid with video games like counter-strike and ghost recon that basically make you an expert if you are good at them. I'm not saying it's the same as real life but it sure prepares you in the head and get's you in the mindset that shooting someone is not that bad.
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