ganime, you need to break baby kitty of that habit or he will still be doing it when he's grown-up kitty. It's cute when they're little, but if you encourage it you will only have trouble later on, especially with a baby.

The best way to try to break him of it is to just stop playing with him, put him down, or walk away from him as soon as he starts to get aggressive. It may take awhile, but he should eventually get the picture. He will also mellow out as he grows out of his kittenhood.

Also, cats are naturally nocturnal, so it's a safe bet he's sleeping when you're working and is not at all lonely.

But if one cat is fun, two is even funner, so I would never try to talk anyone out of getting another!