Like martianbishop, this is a question we've had to deal with. When dd was about 5, a close family friend of our own age committed suicide, and her grandad died (easier to explain as he was old). Grandma has also struggled with cancer for many years. Last year ds's sports teacher was murdered, earlier this year a boy from their school was run over and now their best friend's mum has terminal cancer.
So the line that you die when you're ready is clearly not helpful, as being manifestly untrue. In order to support their friends, my dc's (now 7 and 11) do have to understand and accept that this imminent death is not something anyone feels happy about, it's not because their mum is ready and wants to leave her children. And obviously it would be directly insulting to suggest that a 20-year-old actively wanted to be shot down by her jealous boyfriend.
I think in this situation, they would resent it if I tried to soften things too much. What does help them more is seeing adults suffering and yet trying to meet the situation with dignity- I have pointed out how much my friend does to give her children a special time and how happy people around are to support the family. So in a way, I concentrate on the positives, but I define those as taking the opportunity to show people your love while you can.
I have found it helpful to say that we don't actually know what happens when somebody dies. I've told them that some people believe you go to heaven and that people see heaven as a place where all the things that make you unhappy on earth are taken away ('what? no sauce!!', exclaimed ds, who hates Bolognese). Some people believe you just go to sleep. I've pointed out that their Dad and I hold different religious beliefs, but that death is just one of those things that you can't know about. I have also found it helpful to admit to them that even if, like me, you are a Christian, it still hurts a lot to lose somebody through death.