The "let them die" for me usually comes in when someone is being deliberately stupid, or flat out ignoring mechanics, only to be ressed and do it yet again. No point in wasting a good amount of my mana to res someone that will just die a moment later. But in any other case, I do what I can to keep up with the team. It's not just the healer that has to keep up with the pace, but the damagers, and even the tank him/herself as well, to meet the demands that the current sized mob requires to be met. If people die to overpulling, that alone should be enough to make the team aware that they may have hit their limit a little too quickly, and would then choose to wisen up by decreasing the rate of the pulls and such. It's not often I ever see a tank suffer a complete mental break down and start harassing either the healer or damagers due to an unfortunate wipe, though it is amusing when it does happen.