Clueless

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Here is the question, I doubt anyway can answer this (except God). If you are a religious sort of type, then try to explain it to me. Btw, it is LOOONG!
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Here is the question:
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God said in the 10 Commandments, "Thou shall not kill." but then if you are a policeman/policewoman and you have to gun down somebody who is bad. Is it a sin? Will they go to hell too? But here is the thing, you ARE doing good. Imagine if the world is full of criminals, it wont be good right? And if you dont kill them, and let them free they will start killing again. The bible doesnt state the answer to my question. Also, does that mean all the judges, policeman/policewoman goes to hell? It would be CLEARLY unfair, would it?
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Another one. What happen if you kill ants, mosquitoes or other pest. Isit a sin? Will you go to hell too?
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If possible, anyone out there that is viewing this post. Please reply me. Comment this post, will you? That would help me nicely. Thank you!