I've never spread my "religion" by the sword...as for those who claimed to be following the same Christ that I'm following and chose (or choose) to use a sword to do it, I'd say that I've never accepted such claims as valid. I think the concept of "convert or die" is wholly inconsistent with the message of Jesus.
Note that I am not passing judgment on the salvation of any individual...Jesus made it clear that only God can judge the heart of another person. Instead, I am judging the behavior of people (particularly groups) who claim to be representing "the will of God" against what Jesus instructed his followers to do and if it is not consistent, I'm saying that their testimony about Jesus should NOT be considered reliable (and if it isn't reliable, it isn't valid).
Re: the second part of the question, I'd have to say you're pretty close...it wasn't that people who were actually spreading the message of Christ couldn't get any takers...even when people were using a sword to eliminate his followers (e.g., when the Romans were feeding Christians to the lions or otherwise publicly executing them), they were still able to expand their numbers. The problem was that people who had a "worldly" motive for expanding their numbers (e.g., political power) often seized upon the intuitive appeal of Christ's message as a "hook" for increasing their power enough to be in the position to use a sword to pursue their original motives. A few twists to the words of Jesus and a largely illiterate population and you can mount an army against another group of people led by people who are literate (or just rational) enough to view the revised message as obviously perverted..."convert or die" is the result.
Incidentally, this explanation also applies on a smaller scale...consider all the documented con-men throughout history who put a few twists on what Jesus said to make themselves rich...consider all the people who have "tweaked" something he said to justify slavery, murder, or hatred.
Simple fact, the basic message of Christ has never had any trouble "getting takers". The problem is that the widespread appeal of that message makes it an attractive tool for people to use for personal and/or political gain. That means that those takers are an attractive target for anyone who needs the support/cooperation of other people to achieve their personal/political objectives! It stands to reason that many are going to be unwitting victims of a "false prophet" who convinces them that Jesus wants them to convert people by force. In addition, it also makes sense to think that many more are going to continue to be influenced by the false doctrines that prophet shared with his initial followers as long as the religion he (or she) founded continues to exist. Want some evidence...take a look at all the "Christian" denominations that exist today that hold wildly inconsistent beliefs they all view as "essential" to the definition of a Christian...same basic foundations with inconsistent and sometimes unbelievably complex doctrines built on that foundation.
In a way it's like the fact that Microsoft products are such an attractive target for hackers...the widespread popularity of things like Windows and Internet Explorer increases the power available to someone who wants to exploit others for his or her own purposes if he or she can find a vulnerability.