I don't think we should defend them so much as understand the historical contexts and environment in which they lived. I don't think we should view things through such an anachronistic lens, passing judgment on people whose circumstances were so radically different from ours. It certainly doesn't help anyone to get emotional about people who've been dead for hundreds of years, or more recently than that, considering how even Edgar Allen Poe, one of America's most treasured poets and authors of the 19th century, married his 13-year-old cousin Virginia Eliza Clemm Poe when he was 27.
In that same token, people should have an honest discussion about the Prophet Muhammad and place his life within the context of 7th century Arabia. He was a brutal warlord and did many things, such as mass executions, that his contemporaries had no problems with given there were no international human rights or rules of war, it was the rules of the jungle (or in this case, the desert).
At the same time, it would be wrong of us to judge him for these actions, because in many situations it was kill-or-be-killed and he made executive decisions that were based on ensuring the survival of not only himself but his entirely new religious community of Muslims in a world filled with rival pagans, Christians, and Jews.