That one breaks down pretty easily too. Historically, such bands did indeed exist. However, they did not spend their entire lives killing people. As for repeated killings over a span of time, we have to look not at mass murderers but at real-life serial killers. Once again, we find that reality is not reflected in the way most gamers roleplay.
Serial killers tend to work alone.
They also tend to kill only one person at a time, with individual murders usually being separated by weeks or even months. The online practice of killing multiple victims day after day is once again unrealistic. When we look at the idea of fighting human opponents for the challenge, we find once again that the reasoning of many gamers doesn't stand up. A Quake deathmatch offers a challenge. A duel with an opponent at roughly your level or higher in UO, Diablo et al offers a challenge.