The penalty for leaving Islam is death.
This ‘Law’ has been (And is still) enforced for 1400 years. Part of the problem here is that in many cases, individuals enforce the law. This is in conflict with the idea of governmental enforcement of the laws. Even when governments enact this ‘law’, this reinforces the idea that once something is Muslim, it cannot revert back to anything else. This can be (And frequently is) applied to ‘waters’ and ‘occupation’ of land as well. This concept goes past national boundaries. As a result, this is a nationalistic issue that modern nations have and will go to war over. In addition, this penalty is a major structural problem for Islam.
This ‘law’ is the primary factor in the reason why Sunni and Shiite have not been able to resolve their differences since 690 AD. (Islam was founded in 610 AD) Because the two sides differ in the line of succession of the leadership of Islam, (A major, fundamental difference) each side sees the other as having left Islam. In many of the cases where we hear of ‘sectarian’ violence, this is the basic cause. Historically, when ‘outsiders’ or infidels are involved, Sunni and Shiite will band together to defeat the common enemy before attempting to deal with the other, which is an internal problem.
Execution of people is the sole responsibility of a modern national government, not a ‘religion’. Once again, Islam is in direct conflict with modern governance. Historically, the causes of many wars are of this type. People and nations will fight violently to resolve these types of issues.