The easiest way to encourage cooperative play... is to limit supplies. First, the equipment people start with needs to be reduced. A single can of beans, bottle of water, and pistol with one clip should be all anyone needs for the first hour. Secondly, the spawn rate for guns and ammo also needs to be reduced. Make it a great deal when someone finds a shotgun. Better yet, make them antagonize about spending the oh so rare shells that came with it. These two changes will significantly lower the amount of mindless violence in this mod. It will also greatly increase tears when people well equipped do die, which should please more than a few people. A lot of people forget that survival games aren't just about killing those who threaten the player but also resource management. With resources being so abundant in this mod, managing ones resources isn't that important. This is a problem since it should at least take precedence over player killing.
