Guns are legal in many countries. Hell, I can just go out and buy one (after applying for a license).

And people starting shooting is kind of expected when you break down their doors without announcing. For all they know, their lives are being threatened and they have a right to self defence. But if a cop wants to shoot me here, there is a very clear procedure they must follow (verbal warning+warning shot).

What is wrong with knocking on doors before trying to tear them down?