Ironically I was about to do the exact same thing before reading your post Bob. Even down to substituting D&D with poker.

So what you're saying, kara, is that the prosecution is just bringing up a couple of possibilities for the attack, namely D&D and a girl, and not directly pointing out either one as the real cause?
Yep. The prosecutions job is to make the jury understand why someone would suddenly take a hammer to someone who was supposedly a friend. The prosecution want to paint the picture of a reasoning, nasty person who got pissed off with a friend over a girl and a game they had been playing together and thought that an appropriate response was to pick up a hammer and go over to his place with the intention of murdering him. The prosecution doesn't care what the defendant was doing that pissed him off as long as it can convince the jury that this is a reason a sane man became belligerent.
As Bob's example with poker shows, most juries would buy that someone could become angry enough at losing a poker game to carry out a violent attack so all a competent prosecutor would have to do is explain how D&D can also get people angry, and I don't think any of us would argue it can't.
To be honest, I'd be very surprised if any prosecutor would ever try a D&D made him do it tack. In this case it doesn't appear to be what happened but if a prosecutor did try it, they should be out of their job not simply because they're demonising D&D but cause they're **** at their job and every other prosecutor would see it.
It does no good for the prosecution to paint a picture of someone who was under the control of some insidious board/computer game as that just opens the door for the defence to argue "D&D made him do it, you must acquit. We'll see to it he gets some counselling and never plays again"
These kind of shenanigans are more the sort of thing you see from the defence as they actually can gain from it. A prosecutor who tried it is actually more likely to lose the case as they've just gifted the defence an insanity plea.