I don't understand DarkRP

When I become a policeman, if I kill a mafia guy (who is killing a citizen), I get kicked/banned/no longer policeman.
Why ?

you probably should have arrested him instead of killing him?

The arrest command never works : When I do it, the server says to me “Only the commissary can arrest” or something like that (I didn’t paid attention to that) and it’s doesn’t work.
That’s dumb.
And the time to write “/arrest ***” and the citizen is dead.

Ask on the server’s forum. DarkRP is just a gamemode, servers make tons of modifications and set their own rules.