The toxicity starting to happen.

[LittleImp]

Surely the DPS that pulled the mobs in the first place was the one griefing in this case.

There's a fundamental difference in motivation that's important to understand (This is not an endorsement of pulling as a DPS.);

A DPS that pulls extra mobs is doing so in 99% of cases because they genuinely think the group can do better, and that doing so will make the run more efficient and save themselves time. Although misguided and possibly selfish, they aren't intentionally trying to hurt anyone (usually).

A Tank that intentionally lets someone who pulled die is doing so out of malice, with the explicit intent of hurting another player as a 'punishment', with the known outcome of decreased party efficiency.