I would like to take exception to your assumption. There have been Muslim Rulers and Muslim Invaders just like any other King or Empire. But to say that Islam was spread with a sword is a stretch.
I have a few examples.
1. Egypt - Egypt has been under Muslim Rule for the last 1400 years. Even today, you can find 14 Million Christians in Egypt. If Islam was spead by force, where did these 14 Million (15% of Population)Coptic Christians come from ? Source: Wikipedia.com
2. Spain - Spain was ruled by the Muslims for 800 years before they were driven out. If Islam was spread by sword, do you think that in 800 years, there would be any Non-Muslim in Spain ?
3. India - India was ruled by the Muslims for over 1000 years. But when the greater India was partitioned in 1947, 25% became Pakistan and 75% is what is India today. Think about it, if Islam was spread by sword, don't you think that in a 1000 years there would be more than 25% Muslims ?
4. Indonesia - Indonesia is the most populous Muslim Country. Which Islamic Army marched unto the shores of Indonesia ?
5. Nigeria - Islam is one of the 3 prominent religions in Nigeria. Which Muslim Army marched into Nigeria to spread Islam.
These are baseless allegation. Allah says in the Holy Quran that There is no compulsion in Islam. With some of my %u201CMis-Guided%u201D fellow Muslim brothers, who are using violence, it is convenient for all non-Muslims to label Muslims are violent and also make the assertion that
