As a GM you can have a lot of approaches to combat. The best GMs have several, and vary their approach depending on the enemies involved, the game they're running, and just who and what the PCs are. However, the level of visciousness a GM displays is often closer to a constant. Some GMs play the enemies out as if they were a separate group of players going for the win in a life and death situation. They use every trick possible, and a tactical approach that is easier for a single controller to execute than for a group of separate PCs. Others play more soft, the enemies engage the tanks first and if they can cut through them mayhem may ensue but otherwise the villains play into the way the PCs want to fight.
What about you?
I've been trying to up the viciousness in my own combats. I do not mean I am hunting for my PCs, but I want my enemies to be unique (from each other, not necessarily their kind) and to play and feel appropriately. I want my combats to be more than just rolling dice and applying stats to problems.
Finding the right level however is difficult. Too vicious and it will suddenly be a change in difficulty for my PCs that could be bad for the health of the game. Too little, and I won't hit the tone I need. Still, it is a goal and a lesson to find the right feeling.
But again, how about you?