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Not only was the Robot specifically built for situations like that, but they have continuously been used for decades now, in order to help save the lives of not only the police but anyone else in danger.
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Furthermore, no one has yet to challenge or even attempt to challenge the use of Robots to save lives by killing murderers as being unconstitutional.
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You also said that you feel there may have been a lack of due process here. When someone is "actively" committing a mass