The use of the death penalty is a hot topic with passionately divided opinions, regardless of age, so I'll let you decide that part for yourself. Let's think about the core issue here, which is whether a seventeen year old has the brain functioning to be held responsible for his crimes as an adult would be.
More and more often, courts are saying yes. You have to consider that Simmons could have been only months away from turning eighteen, and there is no magical brain development that happens at that milestone which suddenly makes people reason as adults do. Brains are absolutely still developing in youth until their mid-twenties, but most of this development centers around making judgments from a place of rational thinking rather than a place of emotional reactions. Thus, you see fewer car accidents in the later twenties populations than you do among teenagers. You see fewer late twenty-somethings engaging in risky behaviors than teenagers. Much of this can be attributed to the brain's ability to reason better (and is also likely attributed to further life experience).
I don't think it's possible to say that because the amygdala isn't mature until early adulthood, older teens are completely incapable of making rational decisions. Simmons didn't momentarily have an emotional reaction, quickly reacting from a place of emotion instead of reason. This was a well-planned and premeditated case. He met with friends ahead of time. They targeted their victim. They bound her and tossed her into a river, leaving her to drown. Simmons bragged to his friends about what he had done. He reenacted the crime at the crime scene, which was captured on video.
I would argue that Simmons's actions show very clear reasoning abilities. He did not act in some misguided act of passion, as teens often do with immature brain development. Although horrendous and completely immoral, Simmons planned this crime well and followed it through to completion. Although Simmons was seventeen at the time of his crime and likely had a brain that wasn't yet fully developed as would have been in his later twenties, this crime doesn't reflect immature brain development. It instead reflects a break with moral codes and an appreciation for human life.
Should we consider the death penalty for people who are months away from being legal adults? That's for you to decide. I hope this provides a good basis for your decision.