To be honest I don't know where you have got this from but heres the explaination.
You said any act of violence was permitted this is false. Crops, old people, women, children, and unarmed were never harmed in fact it was prohibited to harm any of these catorgories.
The first war against the pagans of Mecca was fought with a force that out numbered the Muslims 3 to one or more. The muslims at the time had 330 men and only 7 swords most of them had sticks so you tell me does this sound like an organised army. Three years later when the pagans came again, a large trentch was dug to prevent the pagans attacking the muslims. If they were bloodthirsty why didn't they just fight back?
When the pagan were weakened and Mecca was taken from the pagans. What would you have done to nation who had punished you and your companions for nearly 20 years?
The Prophet Muhammad (peace and blessings of Allah be upon him) let them all go and did not kill them or harm them. there was no mass killings against those who plot to fight him.
This was the exception all the other fights were when tribes would arm themselves and be on their way to medina to fight the Muslims. Therefore they had no choice but many people who hate Islam don't talk about this.
The Quran allows self defense it does not allow a muslim to go over the top then they become worse then his enemises.
Intially there was many countires who kill the messengers that the Prophet (peace and blessings of Allah be upon him) and threatened to attack them. This gave them no choice but to fight them. Ask yourself a question why wasn't Abysinna attacked? The answer is simple because they did not want to fight the Muslims.
Many conquests were either not ordered or were unautherised like Tariq ibn Zayad who was on the banks of the alps but the caliph had not given him permission to fight so far into europe. Tariq had defeated the spanish and headed for france in the early 8th centruy. The jews actually welcomed the muslims there, they actually sent messages asking for help against the Christans who where torturing them.
Read Muhammad by Martin lings and you will know what an amazing person the Prophet Muhammad (peace and blessings of God be upon him) is. On the way to conquering mecca there was a dog at the side of the road who was giving birth. Muhammad (peace and blessings of God be upon him) assigned a guard to make sure the dog was not harmed. Does that sound like someone who is bloodthirsty?
Saladin was a great ruler of Islam and re-conquered Jersualam for the Muslims. Just to take you back a little the Christan Crusaders took Jersualam from the Muslims they killed every single living thing, Men, women, children, livestock, babies. There was one story about King james that they brought a Muslim baby infront of them and they cut the baby up and dined on its flesh. So what do you think Muslims did when they re-took Jersualam again? Kill all the Christans there? What do you think? Saladin said there was a gerenal amensity, anyone who wanted to stay could stay and anyone who wanted to leave could take his belongs. Where did Saladin get this from? He took it from his Prophet Muhammad (peace and blessings of God be upon him). Saladin is revered in the west as much as he is in the east.
Also if you look at the places that were conquered by the Muslims they had a re-birth of unprecedented manner.