the old part of old snake is perfectly fine, it's just that his character rolled back dramatically from mgs2 in order to suit the story.
That's exactly what I liked about him. He acted like a real human being. One day you could be the most optimistic person ever, giving advice to everyone about "being happy and smile all day".
And then the next day you can be diagnosed with a terminal disease, and no matter what your previous ideologies were, you might be so scared of dying you may become grumpy or bitter.
We change ideologies and moods during the course of our lives, and sometimes we might not live up to the speeches we make to others. Old Snake, in my opinion, was a fantastic character. He claimed to love life before, but now his life had become a living hell; his body quickly deteriorating, having "seizures", being a soldier too weak for the battlefield. Yet he still kept on going. Not because it was his duty, but because he wanted to try his best to cleanse the world of his "meme" and "genes" before handing it over to the next generation.