Because the trigger runs once for every player. Even if they don't have a DT, it will run for them because the switch is set.
What I'm talking about is this:
P1) No DT at Location, Switch is Set - Centers on DT, Adds 1 to Death Count
P2) No DT at Location, Switch is Set - Centers on "DT", Adds 1 to Death Count
P1) No Zealot at Location, Switch is Set - Center location on "Zealot", Adds 1 to Death Count
P2) No Zealot at Location, Switch is Set - Center location on Zealot, Adds 1 to Death Count
Since P2 has no DT, it will still move the location onto the DT and add to Death Count because the Switch is Set.
My advice is instead of using switches, use "Command" condition.
Current player commands at least 1 DT.