Poisoned by Mamba du Mal snake venom
Uncle Monty, Doctor Montgomery, and Tony "Mommy" Eggmonteror
Bertrand Baudelaire (sister's husband's cousin)
He was a brilliant herpetologist who discovered the Incredibly Deadly Viper. On first meeting him, he gives them homemade coconut cake and the Baudelaires instantly warm to him. Montgomery preferred to be called Uncle Monty by the orphans. He had planned to take them to Peru with his assistant, Gustav, but he had given in his letter of resignation the day before (Gustav was actually murdered by Count Olaf), so he hires Stephano, who is actually Count Olaf in disguise.
The children also watch a movie while in the care of Monty, "Zombies in the Snow" which was a film directed by Dr. Gustav Sebold. The movie warns them about an impostor (Stephano) but no one appears to take notice. The Baudelaire orphans are aware that Stephano is Olaf.
Uncle Monty, on the other hand, simply thinks he's not a very good herpetologist. He rips up his ticket, and is later murdered by Olaf (ironically using snake venom). The Baudelaires expose him, however, Olaf escapes again. The orphans never find a nicer or more caring guardian.
It is strongly hinted that Dr. Montgomery was the keeper of the V.F.D. reptiles and amphibians- creatures that are extremely rare and often deadly, as he found out. As the Mamba du Mal have "excellent communication skills," as the Baudelaire orphans learned, it is only one example of why Dr. Montgomery was keeping them safe. The Autobiography goes on to give us a ripped page from a book called The Mamba du Mal: A Snake That Will Never Kill Me which was written by Montgomery under an anagram pen-name. There is also one part of a V.F.D. meeting that an a person known as "M" is in charge of. As he is normally thought to be at least in his late fifties and more likely in his sixties, we can calculate that Kit Snicket, having her first child, is probably in her thirties (late or not), and she said she was four when the schism took place. Therefore Montgomery would possibly be between 20-40 when the schism took place, although it's more likely he was not in his twenties. It is also hinted that Olaf and Esme are around Montgomery's age (although Olaf and probably Esme, were younger). The Reptiles and Amphibians are an possession both the villains and volunteers want, as Olaf continues to try and corrupt V.F.D. animals to his side such as the eagles and lions.
He is a "short, chubby man with a round red face".