"Mappings": {
    "default": "default",
    "killzone": "killall"
  },
...
  "RuleSets": [
    {
      "name": "default",
...
      "name": "killall",
...
      "rules": [
        "anything can hurt dispensers",
        "anything can hurt players",
        "players can hurt players",
...​

rfc1920

First, make another ruleset (under RuleSets), perhaps as a copy of default (killall in example).  Find and change the line matching or add a new line reading "players can hurt players".  Then add the zone name (killzone) under Mappings:

  "Mappings": {
    "default": "default",
    "killzone": "killall"
  },
...
  "RuleSets": [
    {
      "name": "default",
...
      "name": "killall",
...
      "rules": [
        "anything can hurt dispensers",
        "anything can hurt players",
        "players can hurt players",
...​

I tried what you explained here but I have a problem adding a ruleset.

I did this, which didn't work. The json was validated and still present after plugin reload:

{
  "Config Version": "0.9.7",
  "Default RuleSet": "default",
  "Configuration Options": {
    "handleDamage": true,
    "useZones": true
  },
  "Mappings": {
    "default": "default",
    "killzone": "killall"
  },
  "Schedule": {
    "enabled": false,
    "useRealtime": false,
    "broadcast": false,
    "entries": []
  },
  "RuleSets": [{
    "name": "default",
    "enabled": true,
    "defaultAllowDamage": false,
    "flags": "HumanNPCDamage, LockedBoxesImmortal, LockedDoorsImmortal",
    "rules": [
      "anything can hurt dispensers",
      "anything can hurt players",
      "players cannot hurt players",
      "anything can hurt traps",
      "traps cannot hurt players",
      "players can hurt barricades",
      "barricades cannot hurt players",
      "highwalls cannot hurt players",
      "anything can hurt heli",
      "anything can hurt npcs",
      "fire cannot hurt players",
      "anything can hurt resources"
    ]
  }],

  "name": "killall",
  "enabled": true,
  "defaultAllowDamage": false,
  "flags": "HumanNPCDamage, LockedBoxesImmortal, LockedDoorsImmortal",
  "rules": [
    "anything can hurt dispensers",
    "anything can hurt players",
    "players can hurt players",
    "anything can hurt traps",
    "traps can hurt players",
    "players can hurt barricades",
    "barricades can hurt players",
    "highwalls can hurt players",
    "anything can hurt heli",
    "anything can hurt npcs",
    "fire can hurt players",
    "anything can hurt resources"
  ],
  "Entity Groups": [{
      "name": "dispensers",
      "members": "BaseCorpse, HelicopterDebris",
      "exclusions": ""
    },
    {
      "name": "players",
      "members": "BasePlayer",
      "exclusions": ""
    },
    {
      "name": "traps",
      "members": "AutoTurret, BearTrap, FlameTurret, Landmine, GunTrap, ReactiveTarget, spikes.floor",
      "exclusions": ""
    },
    {
      "name": "barricades",
      "members": "Barricade",
      "exclusions": ""
    },
    {
      "name": "highwalls",
      "members": "wall.external.high.stone, wall.external.high.wood, gates.external.high.wood, gates.external.high.wood",
      "exclusions": ""
    },
    {
      "name": "heli",
      "members": "BaseHelicopter",
      "exclusions": ""
    },
    {
      "name": "npcs",
      "members": "NPCPlayerApex, BradleyAPC",
      "exclusions": ""
    },
    {
      "name": "fire",
      "members": "FireBall",
      "exclusions": ""
    },
    {
      "name": "resources",
      "members": "ResourceEntity, TreeEntity, OreResourceEntity",
      "exclusions": ""
    }
  ]
}

But after the latest plugin update the ruleset part is cleared out in the config json after reload.
I got a zone called "killall" in ZoneManager and also tried with the zone ID.

Can you help me get this right?