Why did Edgar Ross and the government members killed John Marston?

They killed him because he was a former outlaw. He committed many crimes in the past with his "gang" and they wanted to hunt him down for it. I was very disappointed with the ending though. After all, he helped Edgar Ross and his law men capture the gang. But you get a feel for what Marston was striving to do and what he was willing to sacrifice in order to save his wife and son, which adds to the emotional connection you had with him through-out the game. Loved it!

Edgar Ross

They killed John Marston because they had hunted down his gang and he was the last one in it. the government simply used him to hunt down and kill Bill Williamson and the rest of the gang.

Because he was also part of the old gang. They used John to do their dirty work because they had something of his (his family). Then, once he did what they wanted and they no longer needed him, they took him out because he was a criminal and a threat.

they only used him to kill Bill Williamson and when that was done they killed him oh and because he was part of a dangerous criminal gang and they had to kill him(they used him to do their dirty work thats all)