Clearly in this guy's case, his choice to testify wasn't a good idea. There is a presumption that underlies the right of defendants to testify. In the traditional english system, it is assumed they will put themselves in the best light. Remember this dude had time to go get his gun and load it. The jury must have thought people don't think clearly enough to lock and load without time to examine the situation. Still this is a sad situation all the way around. There are no winners in this one.
What makes you so sure it would have turned out any different had he not testified? Look, I don't know anything about this case except what I read in the article. I will say though that I am a criminal defense attorney and I have tried a lot of cases over the years where I have had clients testify because we really felt like we had no choice in the matter. Sometimes you just have to do it to get the evidence out. Sometimes you need to do it because you can see the facts are such that a judge or jury is going to need to hear the story coming from the horse's mouth.
I do not subscribe to the belief that defendants in criminal cases should never testify. I'll advise against it if I don't think the testimony is really necessary, in which case it would be more likely to hurt us than help us, and I'll advise against it if my client has a bad record that will only come in if he gets on the stand. I'll also advise against it if I just think my client will be a terrible witness unless I really need his testimony. Ultimately though, it's their choice whether they testify or not. Sometimes they're screwed if they testify. Sometimes they're screwed if they don't. Sometimes they're screwed either way.