Why did George kill Lennie?

When Lennie killed Curley's wife and ran away, the bunk house men and Curley went on a man hunt for him. Lennie went to the bush that George had told him to go to if he ever got in any trouble. The bunk house men told George that they were going to kill Lennie if George didn't. George knew he needed to get Lennie out of this, and the only way was to send Lennie to Heaven. George went to the bush he knew Lennie would be at. He and Lennie were talking about their dream of owning their own place and Lennie tending the rabbits. George said " We'll have a cow... An' we'll have maybe a pig an' chickens... an' down the flat we'll have... little piece alfalfa-", "For the rabbits," Lennie said. George knew he and Lennie would never get that dream, not anymore. So instead of letting the other guys kill Lennie, he wanted to do it in the least painful and fastest way possible, "He pulled the trigger." and Lennie didn't suffer.