You don't have to be 100% - just beyond reasonable doubt AFAIK.
I mean that the DPP should have the courage to go with the greater charge if he/she thinks that it may stick.
I would have been surprised if a jury didn't convict Sean Keogh of murder, but we'll never know.
Recent case law in the UK would suggest that this pretty much means you have to be sure of someone's guilt. To be honest, I wouldn't convict someone unless I was 100% sure.
I agree that it seems like the DPP takes the easy road but he knows more than I know and understands how he is more likely to get a conviction.
Sean Keogh was found not guilty of murder by the Jury as far as I know. Could be wrong on that though.