This is probably one of my favorite things to do at the moment. In a deathmatch you can go anywhere on the map and not be penalised, unlike Survival where there are boundaries.
Something I like doing is leaving the area completely, and then finding a nice vantage point far away and sniping the other team as they carry on fighting. What makes this work so well is that you aren't on the radar, and it gets even more intense when players catch on to what you are doing and try and track you down.
As a side note, I was once in a 1 vs 6 TDM, all by myself. Once I got the lead by 3 kills I just left the scene and let them try and find me. I think I drove all the way to Paleto Bay, and then started to screw around on the net (probably posting some moronic sh*t here). Then when I got bored, I would go back to the outskirts of the area where we were supposed to fight, and then use Lester to reveal them.
I have only tried this in the shootout area in Sandy Shore and the area where Processed takes place and it was a blast, im wondering if anyone else does this tactic?