悪い is used here as "Sorry" (in a casual way). Don't read it too literally.
買ってくる is a common phrasing when you're going to go out and buy something and come straight back. Again, no need to translate literally when you put it in English. This is quite a common pattern, for example if you wanted to say "I'll go take a look", you're more likely to use 見てくる (not 見ていく). In English we add "go" in phrases like this and the "come back" is usually assumed/implied, in Japanese the "go" part is assumed but you add "come back" instead.