Deathmatch is one of the Modes available in both Blacklight: Tango Down and Blacklight: Retribution.
Deathmatch in Blacklight: Retribution 
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Deathmatch is available on all Maps and is played with up to 12 players, everyone for themselves. This mode is score based as opposed to kill based. Score limit is 4000 points. If scores match between multiple players only then will Kills be factored in to determine the winner.
Deathmatch in Blacklight: Tango Down 
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A screenshot of the Deathmatch mode.
In Deathmatch, it is a free for all, every man is for himself. The player that reaches the kill limit of 35 kills first or who has the highest score (kills) at the end of the time limit is the winner.
It should also be noted that in Deathmatch you will randomly spawn you as either your Blacklight soldier or your Order soldier. Additionally, the color bordering your visor will be either red or blue, also randomly. This however, has no impact as you are not on a team. Kill everything that moves!
There are no Turrets in Deathmatch. Player respawns in Deathmatch generate randomly throughout the map.
Depending upon your Loadout, only your armor type will be affected as it designates you as an Blacklight agent or an agent of The Order as you can have separate Armor types for each Faction. If no armor is set it will default to the Standard armor.
On PC the requirement for a Public Match of Deathmatch is 4 players to start to play.