Hasan promoted radical views, and ended up acting on them, resulting in quite a few deaths of American citizens.

What this has to do with immigration is to illustrate why a very liberal attitude toward Muslims who espouse radical views is a concern for immigration policy.  Who has such radical views?  Typically, it is far more likely for people from radicalized nations to harbor radical ideas that involve sharia law, killing people who disparage the Prophet, etc.   Is there any evidence of this, or is it just based on bigotry?  Yes, there is evidence, viz the problems throughout Europe where Muslim immigrants from radicalized countries are proven to be far more likely to commit acts of terrorism in the name of Allah than are native Europeans.  Is it likely that such problems can happen in the U.S.?  Yes, the already have, ie boston bombing.  Should we just be "Christian" and "American" and pretend none of the above matters?  No, I don't think so.