General GW2 spoilers:Okay I just looked at the vaguest of info, and haven't followed GW2 since *quite* shortly after launch but....
Apparently Balthazar is the bad guy!? Like God of Fire and War Balthazar? And you're trying to stop him from fighting the Elder Dragons? Are they not bad guys now? WHAT HAPPENED
We discovered that killing an Elder Dragon releases their magic into the world. We kill one, and that dragon's magic is unbound and some of it is then fed on by the other dragons, who in turn grow stronger. Then killing another dragon releases their magic and the magic they absorbed from the first dragon. Point being that these explosions of unbound magic from the deaths of Elder Dragons are making Tyria unstable, and the death of a third (we killed two already, Zaithan and Mordremoth) Elder Dragon is likely to tear it apart.
We don't know much about Balthazar's motivations at this point, but we know he lost most of his powers and he is back on Tyria to try and regain them. He absorbed the explosion of one of the Bloodstones (the White Mantle was messing with one in the jungle), but this apparently wasn't enough. He now plans to kill Elder Dragons to feed on their magic when they die.
So we're fighting him because
1) He seems to be completely mad and we don't know what he is going to do with the power he is hoarding.
2) If he kills a third Elder Dragon, Tyria will likely start tearing itself apart from the influx of magic, even if Balthazar absorbs some of it.
We don't know much about Balthazar's motivations at this point, but we know he lost most of his powers and he is back on Tyria to try and regain them. He absorbed the explosion of one of the Bloodstones (the White Mantle was messing with one in the jungle), but this apparently wasn't enough. He now plans to kill Elder Dragons to feed on their magic when they die.
So we're fighting him because
1) He seems to be completely mad and we don't know what he is going to do with the power he is hoarding.
2) If he kills a third Elder Dragon, Tyria will likely start tearing itself apart from the influx of magic, even if Balthazar absorbs some of it.