More than just a bad day

Today at the library I encountered what I think is a genuinely miserable/angry human being.
I couldn't pass it off as more than he was just having a bad day.  I know it's not my business and I will likely not have to deal with him again, but I felt bad for his daughter.
Let me back up.
I asked one of the librarians for her suggestion of some picture books that would be good for Lucy.  Naturally, I'm pretty good friends with almost all of the library staff, and on a friendly basis with all of them since I've volunteered there, so the two of us were having a conversation while she picked out books.
I didn't notice the man standing about five feet away from us with his daughter.  We paused, and he said, "Gosh they're not even paying attention enough for us to ask a question."
Library lady apologized and asked what he'd like to know.  The question isn't important, it was trivial, but it's not like she was just talking to me.  She was helping me pick out books to read to my child, who was peacefully strapped to my chest.
He didn't like the answer to his question (it's a library, not a video game store.  They don't have Wii games, sir) so he walked away with his daughter, in a huff.
As he walked away, I loudly said, "Thank you for helping suggest books for my daughter."
A little later, I was checking out my books, and he asked a little boy near the check outs if you could check DVD's out using self-check out.  The little boy told him yes.
I thought I would be nice and tell him he needed to make sure and unlock the DVD's before leaving, since I had heard library staff telling someone the same thing over the phone (in this case the person had left the library without unlocking them.)
He shot me a dirty look and said, "Thanks!" in the kind of way where you can tell it's clearly not meant.
Both times this guy was a jerk-wad, his little girl, maybe 12 years old, looked bewildered and sad.
I could tell the kid just wanted to have a nice time at the library with her dad and like usual, his bad attitude was leaving her feeling anxiety.
She seemed like a sweet little girl, and I just felt really bad for her.  Dad's are heroes to their baby girls, and I can't help but feel like his behavior is going to rub off on her one day.
Dad's not just having a bad day, he's miserable and angry, and takes it out on strangers by being rude.
In order to fit in with her hero, his girl is probably going to grow up to be rude to others if either he doesn't change or her mom doesn't set good enough of an example to counteract the rude behavior.
I think I notice this stuff more now that I have a baby.  Every little thing I do I scrutinize because I want to make sure and be a good influence for her.  I just hope I don't lose sight of that like this dad has.