In D1, you constantly had access to instant kill weapons like snipers/shotguns and nearly instant kill weapons like the last word (which could originally kill in 2 shots when the hipfire perk was active, and it could be active even while ADS due to a bug that everyone took advantage of. The TTK was faster than normal human reaction time). Teamshotting was a lot less prevalent because you didn't really need it, you could run around on your own and do pretty well. In D2, everything is slowed down and you have a high chance of dying before you manage to kill 1 or 2 people if you're by yourself (unless you hoard power ammo)... Of course people are going to teamshoot. People naturally gravitate towards the fastest way to kill people.