How to explain spreading the DoTs vs aggro management to new players

[Snowaeth]

He was holding agro fine, but was:

A. worried he could lose it, because I had hate from everything
B. Thought I was slowing down the group kill speed by not focusing with our other dps

You were doing the right thing, yet the player was apparently unaware of this.
In such cases I usually suggest them to look it up on the net after the run if they don't want to believe me on my word, in the hopes they will realize their mistake and others won't need to explain it anymore.