A direct democracy, capitalist or not, still goes off people's votes. A direct democratic country like Switzerland could vote for socialist policies and enact them.
You're describing a less corrupt version of the police, which can be achieved through social reform. If the police are held accountable to the law instead of being vindicated for literally murdering black people, abuse would curve dramatically. Even, then you can't avoid killing people. There are some situations like an active shooter where a person has to be killed, and I'm sure the peace keeping force of an anarchist society would choose to kill them as well.
Either way a community-organized security force sounds incredibly stupid. If you don't like someone, you can organize a group of people to kill them and if the majority supported it that would be ok. At least a liberal democracy prevents people from forming literal lynch mobs.