Well think about it like this. You and your neighbor both have guns and know that you can both destroy each other anytime, so whenever there is an argument you are less likely to resort to violence cos you, your neighbor or both could end up losing your lives. But in a scenario where only you have a gun, and your neighbor knows this, he is more submissive to you (Just like how when US were the first to have nuclear weapons, and all the countries feared US). And you are more likely to kill your neighbor in an argument because you know he cannot do you no harm. Its a more extreme case when it comes to nuclear weapons, but the thesis still applies.