The Motivation: Entertainment
The Example:Tom Clancy's Ghost Recon Advanced Warfighter 2
The Crime:
Your elite tactical squad is geared up, synched in and ready to roll. "Here's the plan," you announce. "Confront the enemy on three fronts. Group one, crest the northwest hill and utilize the UAV drone to gather intelligence data on our opposition. Group two, direct the MULE into the hot zone, engage from cover and switch weapon load outs as necessary. Group three, circle the western flank and neutralize combatants with wall-piercing rounds. Everyone coordinate through Cross-Com, stick to strategy and stay--"
Bang, you're dead. "Nice!" shouts your friend. "Did you see the way your arms and legs flailed when I shot you in the face? That was awesome!"
The Justification:
Footballs to the groin and pies to the face aren't considered classic entertainment for nothing. People generally seek diversion that will require as little effort on their part as possible. Slapstick comedy beats sophisticated wit; reality television beats intelligent drama; Disneyland beats the Smithsonian.
So while we appreciate game developers being creative, encouraging us to exercise our brains and pushing us to engage in new, more complex styles of gunplay, we can't completely suppress our base desire to just blow stuff up real good and real fast-like. If the choice is a single kill five minutes from now after lots of strategizing, or five kills this second after some happy trigger pulling, you can't be terribly surprised when some folks go with the instant, dumbed-down gratification.
Yes, team killing is sometimes more fun than playing by the rules.
