Understanding the AWS shared responsibility model is absolutely fundamental when it comes to using and deploying infrastructure within AWS. You need to be fully aware of where your responsibility starts and ends from a security perspective. How can you possibly architect your environment if you do not know where you boundary of responsibility ends? The simple answer is you can't. If you presume that another party, in this case, AWS, is maintaining a certain level of security of your infrastructure, you will almost inevitably leave a vulnerability within your infrastructure allowing a malicious user to take advantage of the weakness and gain unauthorized entry into your environment.
Many users of AWS are only aware of one shared responsibility model, which looks as follows and covers the infrastructure elements of AWS, such as EC2:
