From the BBC:
Because Buddhism exists in many forms, under many organisations, there is no unified Buddhist policy on capital punishment.
In terms of doctrine the death penalty is clearly inconsistent with Buddhist teaching. Buddhists place great emphasis on non-violence and compassion for all life. The First Precept requires individuals to abstain from injuring or killing any living creature.
The Buddha did not explicitly speak about capital punishment, but his teachings show no sympathy for physical punishment, no matter how bad the crime.
An action, even if it brings benefit to oneself, cannot be considered a good action if it causes physical and mental pain to another being.
If a person foolishly does me wrong, I will return to him the protection of my boundless love. The more evil that comes from him, the more good will go from me.
(Via the Death Penalty Information Center)