Mr Santiago is an American citizen by birth- reportedly born in NJ- who killed 5 in Ft Lauderdale, FL.  Mr Mateen, also an American citizen by birth, is the person who killed 49 in Orlando.
You are correct that Mr Santiago's acts , at this point, appear to come more from mental illness rather than as a  terrorist . While there are still folks who would classify Mr Mateen as mentally ill as opposed to a domestic terrorist, neither would have been stopped with immigration laws which is an important  point of not getting hysterical about what we think is happening and spending more time sorting out what is really happening.