Good morning, Christian.
We usually associate cereal with breakfast, which we are often told is the most important meal of the day. As such, it represents essential nutrition: something we need very much.
Rats are associated with sneakiness, and foulness. A rat trying to steal this from you, then, represents someone trying to take away something which you need just as much. It could be your job, or your free time, or a loved one - something you regard as essential.
By killing the rat, you rise to the threat and eliminate it. You have protected (or will protect) that which you need.