Dead is really sensitive and it's STILL not completely clear what triggers him double crossing you. It's best to avoid any confrontational speech at all, INCLUDING skill checks. At the beginning when you first meet him, there's a speech check to say you'll save your questions for when he's done talking. That one is safe. However, there's also a skill check for explosives, for guns, and for merchantile, and it's not confirmed if any of those are safe. He'll take it as an insult to his ego if you seem more skilled than him, so it's best not to take the explosives check, the merchantile check or the guns check (though I think I took the guns check on my playthrough and it was safe). It may also be a good idea not to accidently shoot him; who knows?
Whatever the case, I wouldn't worry so much about being a completionist and sparing his life. Whatever they did to trigger Dean double-crossing the player is certainly more complex than other game triggers we've seen, and I would welcome it fully if in the future, devs were able to develop more game triggers that weren't so easy for the player to decode. It makes each new playthrough with Dead Money an adventure, cause you never know if Dean will double-cross you.
The wiki seems to suggest some generic speech options and the barter skill checks are his hot button:
His ego is very fragile, and the player is encouraged to take care not to bruise it when dealing with Domino, lest it carry dire consequences down the line. Expressing irritation, aggression, or contempt will earn his ire. Also, passing the Barter skill checks in his dialogues will turn him murderous later, as he takes the player's competence as a personal insult. Skill checks other than Barter checks are okay, as they are the only ones that will have a negative impact.
» Fri Nov 18, 2011 10:52 am 
