The totally sad and sorrowful thing about the tragedy with Lennie Small and Curly's wife in the novel 'Of Mice and Men' by John Steinbeck is that for once, Lennie was a 'good boy' in doing what he was told! George's earlier scheming and rehearsing with Lennie about what to do and where to go if there was any more 'trouble' worked like a dream. Lennie obediently followed George's advice as he was asked to do, and so George knew exactly where to find him in their 'safe place.' Lennie's learning challenges made him unpredictable, difficult and obstreporous sometimes which made him disobedient - and as passive as a lamb at other times. Sadly, When George sprung into action and went on ahead to find Lennie after the killing, he must have been hoping this was one of Lennie's obedient moments.
George begins to plan right away.
George sneaks out the bunkhouse right away and snags Carlson's gun. Everyone else kind of looks around and tries to make something of what happened and Curley immediately jumps to the conclusion it was Lennie and vows to take him out.
George goes a way other than the men are going so that he can catch up to Lennie at their 'spot' at the tide-pool next to the river they camped at the night before the started work. George had done this just so they'd have a meeting place if something went wrong.
George's reaction was definitely shock, but then an accountability to what he needed to do to take care of the situation as responsibly as possible.
George's first reaction is one of shock. To be precise, he says "Jesus Christ." From there, his reaction is to figure out what to do. His main goal is to figure out what would be best for or nicest to Lennie. Of course, he ends up deciding that the best thing is for him to kill Lennie himself.
Other than that, he also reacts by thinking about what this means for his dream. He understands (with Candy's help) that this means that their dream of having the farm is over. He also reacts by talking about how maybe he knew the dream never really was possible.
In the book "Of Mice and Men" George immediately knew that Lenny had to have killed the girl when he saw her. He knew right away that he could have to find Lenny before the other men did. He told the others what he had to do and he took off to find Lenny.
Once George found Lenny he realized that he would have to kill him to prevent other from being brutally hurt by Curley and the boys. He takes the gun and while talking with Lenny about the farm they will have together and making him feel safe, he shoots him in the head. Lenny dies instantly.