That's exactly right. You do have to ask yourself that. But how do you know what is going to cause this man more harm than good? The news is going to be tragic regardless of when you break it. You make a good point about the timing of breaking bad news being important, but you also make a lot of assumptions about what will happen when you tell him (wanting to die, refusing treatment, etc). (As an aside, emotional distress =/= incompetence, and you cannot "force" a treatment upon anyone who is competent.
I don't think basing the decision on whether to lie or not should be dependent on hypothetical future actions by a competent adult. You could very well wait a month to tell him and he'd still go out and commit suicide. All you can do is try to create as supportive an environment as possible and do your best to help him through it. I agree that oftentimes in the ED is not the ideal place, but if he is with it enough to ask the question, he has kind of forced your hand and you do the best you can. You can try deflecting, "why don't we get you taken care of first and then we can worry about your wife," or "unfortunately we have bad news, it might be better if we do this with family and friends around," but if I were in the patient's situation, I doubt that would cut it and I would want to know. And if I found out you had lied to me, I would be pretty peeved.
The take-home point is, it is dangerous to assume that you know what is best for a patient emotionally, especially as young doctors (and soon-to-be-doctors) with mostly limited life experience.