Leaving aside the way you seem to be implying that people who don't want their guns registered with no legal authority are likely to be murderers...
It's about principle - if it's fine for the police to act outside the law when they think something is for a good reason, where do you draw the line? Is it ok for them to tamper with evidence because they're pretty sure the accused did it? Or lie about their actions because the truth is inconvenient? Some of them do that too, are you saying that's ok because they're police? Hey, maybe it's ok for police to shoot someone just because they think they might be a bad person? After all, if you don't give them a reason to think that then you have nothing to worry about.
The law is there for a reason and the police are subject to it for a reason too, it's not up to them to change it or break it when it suits them.
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