Jekyll is killed by suicide. The author, Robert Louis Stevenson, crafted the finale such that it is unclear if Jekyll is still in charge at the end or if Hyde is the one who commits suicide. However, because they are the same physical creature, when one dies, so does the other. Therefore, Jekyll must commit suicide to ensure his death. There are several options for how this could happen. One possibility is that Jekyll could have an accident and die. But because he's a doctor, it wouldn't be considered a true suicide. So then Hyde would have to take his own life to keep Jekyll from coming back to life. This could be done by hanging himself or using some other kind of fatal means. Another option is that Jekyll could choose to kill himself. But because he's a doctor, he wouldn't consider it a real suicide and so would not be able to go straight to heaven or hell. Instead, he would have to wait until after his body died before entering either realm. With this in mind, he could choose to end his own life with a lethal dose of drugs or even get into a car crash while driving drunk which would cause him to die. Then his soul would be free to go where it wanted. Yet another option is that Jekyll could decide to live forever. But since he's a doctor, he would know that there is no way to do this naturally.