Killing cops is considered such a severe crime because it is the first step of any revolution to remove the police from an area in order to claim its independence. Without this first event, a civil conflict cannot happen.
Police officers are the front line manifestations of government power, they are the front line troops of any government, they are the most intimate representatives of government to the people, to get to the people who run the government, you need to go through the police, these are first line of human shields protecting the government from the people, these are the conduits through which government power is discharged and through which societal order is maintained.
To kill the police is to reject the authority of your government and to challenge its legitimacy.
It's getting past the first layer of your society, to take on the most external circle of government, is required to get progress towards its core.
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