First past the post protects criminals

We can assume that without a government there will be crime because by definition if crime is prevented by an authority then that authority is the government. Only the government can 'officially' stop crime... if an individual on their own stops crime then this is not inconsistent with anarchy but to have an official body do so is to have a state. The government (democratically elected) is the means by which the people are able to collectively protect themselves from crime. It is as well the means by which the people are able to endorse a police service.

Without democracy and the government (in an anarchist society) it is much easier for criminals and criminal gangs to prosper because the natural defensive and collaborative behaviours of the group are removed. A democratic government is a natural expression of the (peaceful) desires of the group. So without democracy there is less opportunity to prevent crime.

First past the post is much less democratic than other possible forms of democracy and because crime is prevented by democracy this means fptp helps crime. It is worse for criminals if the people are able to fully express their intentions via the ballot box because overwhelmingly they are opposed to crime. They might disagree about some aspects of government policy but they are more-or-less united against crime.

Because first-past-the-post damages democracy and government it damages the ability of the people to prevent crime.