Sorry to disagree. But that is not true.
What muslim countries do you refer too?
According to this site, http://encyclopedia.thefreedictionary.com/honor%20killing "In Indonesia, the world's largest Muslim country, honor killings are unknown, as also in Muslim parts of West Africa."
It also points out that honor killing has been indulged in by Christian and Jewish inhabitants of Arab countriess, as well as by Sikhs and a number of other religious groups.
Honor killing is practiced by some non-Muslims, and some Muslims, including the largest Muslim nation, do not practice it.
It is likely that the practice is much more cultural than specifically religious, although its less than educated practitioners are probably unaware of this.
The Koran specifically requires four adult male witnesses of the actual act in progress before a woman can be convicted of and punished for adultery, and then the punishment comes from the state, not the family.