This is factually wrong in every respect. Just over 50% of the states in the world that are UN members/observers have abolished capital punishment completely. Of these, several of them, such as Mexico and Brazil, have some of the highest rates of murder per capita in the world. In fact, Honduras, which bans capital punishment, has an intentional homicide rate of 90.4 per 100,000 people, the highest in the world (it's nearly double the very next country down, Venezuela, which has
also banned capital punishment). So no, there are not ten countries with lower murder rates for every country that supports capital punishment. I don't think the ratio is even as high as one-to-one.
Also note that more than 60% of the world's population live in a country that supports capital punishment. The four most populous countries in the world, China, India, The United States, and Indonesia, representing approximately 44% of the world's population, all support capital punishment and have a per 100,000 murder rate of 1.0, 3.5, 4.7, and 0.6 respectively. China, India, and Indonesia are well into the bottom half of the list of countries by murder per 100,000 people.
It seems to me that trying to base your opinion on capital punishment by looking at murder rates is not going to do you much good given the huge variation in murder rates vs legality of capital punishment.
References:
https://en.wikipedia.org/wiki/List_of_countries_by_intentional_homicide_rate (UNODC references are linked within the article)
https://en.wikipedia.org/wiki/Use_of_capital_punishment_by_country (Same as above)