Some are picky but even those you can almost always train them to eat pre killed. Usually the most you need to do is just warm the rat up and wiggle it with a tongs. Snakes are pretty dumb so that’s enough for it to look alive to them. It’s a whole lot more convient to have a freezer full or rats than to keep live rats. Not to mention live prey can be dangerous to the snake; it usually isn’t but before I finally got my snake to start eating prekilled one of the live rats I fed her gave her a scratch on her eye cap.