Spoiling your monster involves giving into it whenever it wants to be lazy and skip Training, only feeding it food that it likes and giving it unnecessary gifts.
In some games, monsters will develop a nasty Trait called Willful that will keep it from listening to your commands in battle. In other games it ensures that you cannot raise your monster's Loyalty stat beyond 50 points.
You can balance out a Spoiled monster by instilling it with Fear every once in a while, disciplining it when it disobeys and being kind and fair in your choices. This will give you a balanced Raising Style.