The death penalty is a very effective crime deterrant. Recently, the Singapore government exercised the death penalty to Van Tuong Nguyen for bringing drugs into Singapore. I am sure those who were contemplating bringing drugs into Asia (especially into Singapore) at the time of Van Nguyen's trial gave second thought to the consequences of drug tafficking, and decided in the end that their lives were too valuable to risk being caught trafficking drugs.
The purpose of the death penalty is to warn others bent upon the same road. Some heed the warnings, and some don't. The death penalty is especially effective as a crime deterrent for children. Children are easily impressed, and examples of harsh penalties are very effective for teaching them. The law without visual reinforcement is like a parent who threatens but never fulfills his/her word. The child will soon learn to disregard the parent and break all rules. It is the same in the world. If the death penalty is punishment for breaking a certain law, and a person breaks that law, the death penalty MUST be imposed. Children will see that the law means business, and will hereafter tread more conscientiously.
Of course, there will always be the few odd people who rebel and dare to put their lives in danger by committing acts they know to be punishable by death. But these will be far and few if the death penalty is strictly adhered to. The death penalty is a powerful and effective crime deterrant, the highest form of punishment man can apply.