Indiscriminate violence against groups

Greece migrant crisis: Islanders strike over crowded camps. BBC, 22 January 2020.

"Strikes and protests have been held on the Greek islands of Lesbos, Samos and Chios over the government's handling of migrants arriving from Turkey.
Banners on the Lesbos municipal theatre proclaimed: "We want our islands back".
Another read: "No more prisons for human souls in the North Aegean."
North Aegean Regional Governor Kostas Moutzouris said on Wednesday he was "annoyed" that Greek islands had been "turned into places of concentration and detention" for thousands of people around the world.
In Samos town, the refugee camp is in the olive groves on a hill just a few minutes' walk from the town centre.
It is common to see migrants hanging around on benches at the seafront.
Migrants live in tents at the camp on the Greek island of Samos."

Why haven't locals taken matters into their own hands? Locals should form a secret society to shoot migrants landing on their shores as soon as they step off the boats, if not before.

Similarly, a reconquista of the streets requires a secret society to attack members of high-risk groups who enter civilised areas, even before they commit crimes. This is criticised as unjust, punishing the collective for the actions of individuals, but it is effective and has been used successfully historically. In the above case, it is not even a response to actions, but a means of holding territory.

Similarly, reprisals against a group are an effective means of deterring individual crimes.