I'm curious to know where you got the idea that "twelver" is a derogatory term.  I've used the term many times on the www.shiachat.com forum, and I can't recall anyone being offended.  In fact, for over ten years at work I shared an office with a Twelver of Iranian ancestry, and he never indicated to me that the term Twelver is an insult.

 

Basically, the term "Twelver" refers to the fact that this sect of Islam believes that there were 12 divinely chosen leaders after Mohammad.  See, for example, the following website for more details:

 

 

If you personally know of muslims who consider it an insult to be called Twelvers, it would be interesting if you could invite them to add their posts to this thread.

 

 

 

And, it is common for Sunni suicide bombers to target Shiites.  For example:

 

 

which reads, in part:

 

 

You need look no further than this forum to uncover Sunni muslims who are critical of Shiite beliefs.  For example: on January 18, 2013 in the thread "The Final Hurdle to Becoming a Muslim is", Abu Loren wrote:

 

Depending upon which sects you personally consider to be muslim, I don't think your statement is correct.  People calling themselves muslim differ greatly on the fundamentals of Islam.

 

For example, approximately 200 million of the world's muslims are Twelver Shiite.  Twelvers believe in praying three times a day (instead of the five times a day Sunnis pray), and add an additional clause onto the Shahada:

 

 

A question to ponder: do either of these differences make Twelvers apostates of Islam?