It remains to be seen whether or not the war would be "lost" if the Fire Lord died. If Zuko is a branded traitor along side Iroh, the only person remaining would be Azula. That's not going to stop the war.
Anyway, I'm not convinced that "losing the war" and "being against the war" are two different things. Zuko has seen what the Fire Nation has done and I think he now stands against the war because the fire nation isn't spreading "culture and prosperity" but is just out to rule the world. I think he's now, currently, against that philosophy and thus the war. If Zuko wanted the war to go on and take over the world, he would have just killed his father and usurped the throne, and then killed his sister with his new position. I'll say that this is just my opinion.
The same applies for Zuko's goodness in a sense. If he's siding with good, he is essentially siding against the war and against the fire nation's current form. If he isn't against the war, then there isn't much reason to his actions unless he's going to use the Avatar to decapitate the government (which wouldn't make any sense if he had a chance to off Ozai in the first place). And, I'm sorry to say, I don't think Zuko is much of a long term planner. If he wanted the throne to lead the war, he would have killed Ozai and taken it, not have a long plan of using the Avatar as a weapon. It's just not how he's been shown to operate.
Again, my opinion.