Well, yeah, he's getting better. Look at his character progression:
1) He begins his life in a mercenary band. He's treated horribly by his adopted father, but he seems to "love" him all the same. However, things go wrong, and he kills Gambino. He is driven away. Guts loses his "family".
2) He continues on, always alone. He fights because it's all he knows. He finally meets the Hawks, and if defeated in battle. He joins the Hawks, and comes to regard them as a new "family".
3) He leaves the Hawks, because he feels that he has to seek out his own dream. He trains for a year and returns to his "family".
4) Guts rescues Griffith. As the Hawks are deciding what to do next, the Raiders approach him and ask to come with him. Guts finally realizes that he has a place where he belongs. He is content for the first time in his entire life. On top of that, he has Casca, whom he loves.
5) Boom. Eclipse. No more family. Guts loses everything, right after finally realizing what he had. You can imagine how horribly this affects him. His entire life changes...now he has to spend his nights restlessly, constantly on the move, fighting demons at all hours.
6) Quite a bit of time passes this way. Guts is always alone, living in darkness. You can imagine the sort of effects this had upon his mind. It's understandable that he's a cynical monster when Puck meets him.
7) Puck is the beginning of Guts' change. Guts goes through the Lost Children arc, but then the Casca stuff happens. Guts realizes that he has to protect Casca. No longer is he fighting for the sake of vengeance, instead he is fighting to protect the woman he loves.

Guts gradually forms a group of followers. Now, finally, Guts is regaining a "family".
It's only understandable that he's becoming kinder.
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