And then Walter arrives with Astrid, and opts to go upstairs instead of in the other direction to the conference: "We need to take the higher ground," he "explains" to Astrid.
While the conference speaker continues to drone on, the FBI crew keeps searching, finding nothing, and Olivia just misses spotting Hoffman as he strolls through the crowd. Peter, meanwhile, is over in the bar: "It could be anything back here. Candles, coffeepots ... it's a disaster." Then he spots something, and Olivia rushes to join him. He intercepts a waitress just as she's about to light one of the canisters. "Let's hold off on those for one second," he says, grabbing the waitress's hand. Fortunately she's a waitress who amiably listens to an order from a complete stranger instead of just doing the job she's paid to do. Peter grabs the canister and sniffs it, as Olivia arrives.
And then they hear some coughing and start racing to the conference floor, where one benefit of the coughing fit is that the speaker has stopped talking. Peter gets to the victim first: it's Hoffman, turning a greyish-blue on the floor. Peter recognizes him, and stares as Hoffman chokes out, "Bischoff! Traitor!" and dies. Peter looks up, behind him, at his dad in the balcony. Walter's standing there with smoke drifting out of that little device that he snagged from Hoffman's lab. And everyone is just standing there staring up at Walter, and nobody moves or does anything or freaks out, despite nobody having any idea what the hell's going on. They must all be numb from listening to that speaker for so long. "Hey, someone just coughed themselves to death, and yelled something about a traitor, and there's some dude up in the balcony holding some strange-looking device with smoke coming out of it, think I'LL JUST STAND RIGHT HERE AND DO NOTHING."
Afterwards, Broyles glowers at Walter: "What you did tonight--" he begins, and Walter finishes: "--was to use the killer's DNA to target him and only him using his own toxin. If you plan to press charges, then so be it. But I don't regret what I did." Broyles stares at him for a minute before sighing. "Good night, Dr. Bishop," he says, and walks away. Probably doesn't want to be Walter's NEXT VICTIM.
Walter snags Olivia so he can explain himself, sort of. "That man... he, he corrupted my father's work." Olivia says she understands. Walter says it's more than that, though: "You see, family is very important to me. There's nothing I wouldn't do." Why doesn't Walter just walk around with a sign that reads, "Someone please ask me about the time Peter died and I made a clone of him" already?