Why is Roland convinced that Billy Pilgrim is responsible for his death?

The above answers are the same. Roland did blame Billy for his death.

Roland Weary died--of gangrene that had started in his mangled feet. So it goes. Weary, in his nearly continuous delirium, told again and again of the Three Musketeers, acknowledged that he was dying, gave many messages to be delivered to his family in Pittsburgh. Above all, he wanted to be avenged, so he said again and again the name of the person who had killed him. Everyone on the car learned the lesson well.
'Who killed me?' he would ask.
And everybody knew the answer, which was this: 'Billy Pilgrim.'