What's the best way to make NPCs that attack players?

Right now I’m using this code, but he only follows and attacks the player after he sees him.

local ent = ents.Create( "npc_metropolice" )
ent:SetPos( cop_spawn_locations[1] )
ent:SetKeyValue( "additionalequipment", "weapon_stunstick" )
ent:AddEntityRelationship( player.GetByID( 1 ), D_HT, 99 )

I want to have an NPC that will actively seek out and kill the player wherever he is. What’s the best way to do this? Do the HL2 NPCs have code for this or do I have to make a new custom SNPC/Nextbot?