Majority rule does help achieve peace. In order for this to work effectively, people have to have a similar understanding of what peace is and discuss issues together to come to a peaceful resolution. Not everyone will necessarily agree with this theory, but it has worked in the past even when certain conditions have to be put on majority rule.
In any society, peace is fragile and must be cultivated. Peace has less to do with what type of governmental system is in place, and more about the core values of a particular society and those governing it. If a country and its leaders want peace, it is more likely to be peaceful irrespective of its political system.