How do you convince Dustil? You don't want to do it the way they told you. The way I did it the first time is far more interesting and rewarding (in more ways than one).
It wasn't my first play-through of K1. This was the first play-through that I took my time to get more of the quests in (except on Manaan, of course). Since I heard later that Dustil was in Korriban if you talk to Carth enough about his [ridiculously stock] past, I endured through the [agonizing] conversations with the pilot and got admitted into the Korriban Academy, taking Carth and Jolee with me.
It's not long before I find Dustil in his "room" which doesn't have a door (there also seem to be no restrooms in the academy). Anyway, I click on him and discover to my unending amusement that Dustil is voiced by the same guy who was Rosh Penin, another garbage character that LA made.
Once I get over this, Carth and Dustil have their little reunion, Dustil whines that Carth abandoned him, Carth whines that he did all he could, yadda yadda yadda. Since I'm a lightsider this time around, I attempt to convince Dustil that the Sith are evil (which is a very hard thing to do because of my naturally intense hatred of Carth and anyone who he's related to). Dustil, staying true to the typical "Onasi Whiny Arrogance" traits that were obviously passed down by his "father", tells me to piss off.
I press the issue, and Dustil draws his lightsaber and attacks. Ok, that was lame. I have to reload and talk to that brat again. Repeat, only this time I don't bother him again. I somehow get the idea (possibly from a conversation or my journal, don't remember) that I need evidence of the Siths' Evilness. Therefore, I resolve to find something (probably a datapad) on the way. I go about numerous quests on Korriban, and I find the datapad in Uthar's quarters (among a few "novelty items" I'd rather not discuss). Since I'm in no rush, I continue the other stuff on Korriban, and before I know it I've already forgotten the story of Carth and his confused, troubled son (yawn).
Anyway, I eventually get to the point where I kill Uthar and his apprentice. Even I can remember that this doesn't go over too well with the rest of the academy, so I summon Carth and Canderous from the Ebon Hawk, buff up as usual, and prepare for battle.
Needless to say, the retards at the entrance from the valley stop me and demand to know where Uthar is. When I tell that that he's sleepin' with the fishes, they go berzerk. Seeing a window of opportunity, I reveal the truth of my identity, but, like everyone else, they refuse to believe that I'm Revan (did I wear that mask my whole life, or did everyone just forgot what a great war hero looks like?). It's kind of a running gag, nobody believes me even though I'm telling the absoloute truth.
After wasting the morons with ease, I enter the academy, ready for anything. After entering, a swarm, a freakin' SWARM
of students make a bee-line for me, weapons raised (apparently the Sith are all omnicient [spl?] and therefore know everything that goes on in the academy). It takes a while for me to chop my way through them, but I manage. However, as I make my way down the hall, it suddenly occurs to me that the Sith ALL turn hostile. However, I don't worry about the Dustil quest, since the game wouldn't leave a hole like that where a quest is impossible to finish due to a totally unrelated set of events.
I fight my way to Dustil's room. Dustil charges me with lightsaber armed. Well, this is kind of unfair. How do I get my XP? I've got the evidence RIGHT ON ME, I can SHOW IT TO HIM NOW!
Anyway, I fight the punk for a bit, who puts up a ridiculously long fight, and just as his health gets down, Carth lets loose a torrent of blaster fire into Dustil's chest. Needless to say, he doesn't get up.
I talk to Carth and all he says is some mumbo jumbo about him not being able to let the fact that HE JUST KILLED HIS OWN SON get in the way of the mission. He has to grieve about it later.
The fact that this is Carth's reaction to the fact that he murdered his own son in cold blood proves my theory. Carth just made up the story about his wife. He never had a wife. Carth's son was adopted. There's no other possible explanation.
Needless to say, I pissed my pants laughing.