I don't understand why they locked him in a room with nightlock (kills you fast) what did he do?
Katniss and Peeta defied the rules of the game and made Snow and the Capitol look weak.
Seneca Crane was the Hunger Games Master and the public face of the failure, and also the one who changed the rules of the Hunger Games in the first place, allowing Katniss and Peeta to make their stand together. Ordinarily, even the tributes from the same district must turn on one another. But because of Seneca's attempt at crowd-pleasing, Katniss and Peeta get their chance to die in defiance of the Capitol.
President Snow blamed Seneca for this. Snow knew that the public defiance of the Capital would only lead to further uprisings, and set in motion his attempts to spin Katniss and Peeta as a pair of desperate lovers, rather than headstrong rebels.
Snow devised the punishment as a paradox: to eat the poisoned berries for a quick and sure death, or wait with no escape and no other sustenance to either starve or spark the further displeasure, which could lead to a more painful death. Snow has a twisted sense of justice and a thirst for blood and vengeance. He views the death trap as a poetic or symbolic punishment, itself embodying the Hunger Games Seneca failed to conclude properly.
Seneca Crane was the head Gamemaker until after the 74th Hunger Games.
About halfway through the games, he had announced that there could be two victors if they were from the same district. Katniss and Peeta saw this chance and took it, but once they had killed Cato, Crane announced that there could only be one victor. His plan all along was to have Katniss kill Peeta or vice versa, because that's what the people in the Capitol love, but it went awry when Katniss suggested a double-suicide.
There had to be a victor, so Crane allowed both of them to win, but Snow wasn't pleased. He had seen Katniss as the leader of the rebellion as soon as she had suggested the suicide instead of killing Peeta, so he wanted to take it out on someone. That someone was Seneca Crane, the man who had let the two tributes win. He locked Crane in a room with nothing but a bowl of Nightlock - the berries that Katniss and Peeta were going to eat for the double-suicide. His only choices were to eat the Nightlock and die almost instantly, or to die of thirst and hunger. He chose the nightlock so that he could save himself from a painful and long death.