There is a suggestion in the play that Chris always knew about his father's guilt. It is hidden in the fact that Chris has never allowed his name to be put on the business, and in the remark that everyone in the town knew that Joe was guilty. (I wish I could provide you with the exact references in the play, but I don't have a copy here.) Chris lived with what he knew/suspected and might have spent the rest of his life in the nowhere land until Annie and George came back and Joe, in explaining what happen to George, made the mistake of stating that he had never been sick --- a direct contradiction of what he said when he passed the blame along to Steve.
At that point, it is clear to Chris, idealist that he is, that Joe is guilty and that he needs to assume responsibility for what he has done; at this point, though, we do not know that Larry killed himself because of his shame at what his father had done. When they all read Larry's letter, Chris becomes increasingly incensed at his father. For his part, Joe comes to realize that he is responsible for the death of his son, part of the family that he claims he has always acted for, and goes into the house to commit suicide.
At that point Chris seems to realize what "he" has done, and the play ends with Chris in Kate's arms and many viewers wondering if Chris really did the right thing ....
In the beginning of the play, Chris believes that his father is innocent. He trusts that his father was exonerated because he was not guilty of the crime and Steve Deever was guilty. Chris believes in the system, he is idealistic in his view of life, he trusts that people, especially his father, would not lie.
That is why when Chris finds out about his father's guilt, his deception, he is stricken, sick, he runs off. He can't bear the thought that his father is dishonest, he equates his father's mistake in judgement with a lack of integrity, saying if Joe has no integrity, then he has no integrity. He feels dirty by Joe's behavior, he feels betrayed at the deepest level.
Chris is still in shock, absorbing the fact of Joe's guilt; he is so affected by this news that he feels that he must leave the house, go into a self-imposed isolation away from the neighborhood, away from Ann. He feels so changed by the news that when he hears that his brother killed himself over the discovery of Joe's behavior, Chris understands, this is expressed through his own intentions.
Like Larry's fated decision to end his life, Chris will end the life he knows in reparation for the deaths his father caused. At the end of the play everything happens so fast. Before Chris has a chance to truly react to the news, Joe Keller shoots himself.