Luckily I have no experience with people who delivered bad news in bad ways. I have been the messager myself, and I did a good job though it wasn't easy in one particular case.
I was in the hospital after a call from my BIL that something had happened to my sister. I went over there and got the big bad news from a police officer. I called my mother to say that I'd be over at her house for something important, so I could tell it face ot face. I figured I would let brother come over too, for the same reason.
I call him and tell him he should come over, there is something about my sister. Now, my sister has been in the hospital before because of an overdose (no, not an accident). My brother thinks its something like that, that she is on the stomach pump again and gets angry at her and me, and even starts shouting. Luckily, I managed to stop him in his stacks before he manages to say anything bad, and tell him to just get over there. Which he luckily did. He had heard from my mom already in the meantime, and was a sobbing mess when he tried to apologize to me.
Maybe my brother sounds like a bad guy here, but he had already raced to the hospital twice before in three years before that, because of voluntary overdose from my sister. He is quite hot tempered and jumped to the wrong conclusions - but then again, who would expect a death call on any given day? he apologized and meant it, and I wasn't even upset with that in the first place (had other things to be upset about) so it was fine.
And yeah, close relatives who have to relay the big bad news should be given a lot of leeway. It's not something you're trained in, and with a head full of grief it's easy to blurt it out all wrong.