It may shatter your worldview to hear, but many people don't actually own a true "butter knife". If it has a serrated edge, it's technically a table knife, or possibly a bread knife. Don't let that stop you though. To spread butter with it more easily, flip it over and use the flat side.
Bread knives have serrated edges to make cutting bread and soft meats easier. If you use it to spread butter, you can end up tearing the bread. Hard to fault the knife for this, since that's what it's supposed to do! Instead, flip the knife over and use the flat edge. This much more closely resembles what a proper butter knife is supposed to be: flat, non-serrated and curved at the top.