What was the reason behind emergence of the dark wizard Lord Voldemort? Why Tom Riddle a student of Hogwarts had to become Voldemort? What is the reason behind such dreadful appearance (flat nose) of Voldemort?
He became a dark wizard when he was in Slytherin, and since he was named after his father muggle he decide to change his name using an anagram "Tom Marvolo Riddle" -> "I am Lord Voldemort"
The transformation into his monstrous state is believed to have been the result of creating his Horcruxes and becoming less human as he continued to divide his soul
His nose shape is due to his relationship with snakes
I believe Voldemort's snake-like nose is a response to the fact that one of Voldemort's temporary bodies was created by Unicorn blood and Nagini's venom, as per:
Wormtail was able to follow the instructions I gave him, which would return me to a rudimentary, weak body of my own, a body I would be able to inhabit while awaiting the essential ingredients for true rebirth … a spell or two of my own invention … a little help from my dear Nagini’ – Voldemort’s red eyes fell upon the continually circling snake – ‘a potion concocted from unicorn blood, and the snake venom Nagini provided … I was soon returned to an almost human form, and strong enough to travel.
Harry Potter and the Goblet of Fire
Tom Riddle has always had darker tendencies. Ever since he discovered that he had otherworldly abilities, he had been manipulating and terrorizing the children at his orphanage. When Dumbledore visited him at his orphanage with the offer to join Hogwarts, he said
“I knew I was different,” he whispered to his own quivering fingers. “I knew I was special. Always, I knew there was something.”
And later Dumbledore told Harry that
"In fact, his ability to speak to serpents did not make me nearly as uneasy as his obvious instincts for cruelty, secrecy, and domination."
So it really was not a matter of Tom Riddle having to become Lord Voldemort. It was more symbolic of his intentions and of the belief he had in himself that he was greater. He had always been that cruel.
As to the other part of your question, his appearance can to attributed to the repeated creation of Horcruxes and his propensity for snakes, as others have pointed out!