No. Breaching a contract is not illegal. If you are sued for breach of contract, and a jury (or judge in a bench trial) finds against you, you are found "liable," not "guilty." This means that you have to pay the damages that the other party suffered as a result of your breach. It does not mean you have committed a crime. The legal doctrine behind breach of contract is merely about making people whole by repaying the damages that result from breaching the contract. In fact, parties to a contract often make a conscious choice, after performing a cost-benefit analysis, to breach a contract, and this is perfectly legal.