I intially put his long standing Islamophobia down to his upbringing as some Assyrians and Coptic Christians are indoctrinated to dislike (hate) Islam from youth. Unfortunately, Sam has been unable to curb his rebellion and resentment towards Muslims despite being in his 40s.
It seems as though Sam Shamoun's problems are in fact deep routed. In his younger days Shamoun dabbled with the ideas of the racist black supremacist group known as Nation of Islam (NOI).
The teachings of this group would appeal to rebellious and disaffected youth. This group is extremely racist towards White people.
The official beliefs as stated by the Nation of Islam have been outlined in books, documents, and articles published by the organization as well as speeches by Elijah Muhammad, Malcolm X, Louis Farrakhan, and other ministers. These include inflammatory statements as well as the pejorative use of the term “white devils” to refer to white people 
Louis Farrakhan has stated that "White people are potential humans…they haven’t evolved yet." (Philadelphia Inquirer, 3/18/00). 
Sam Shamoun, if I am not mistaken, left Christianity for the NOI and stayed with the NOI for FOUR (?) years. Four years (?) is along time for black supremacy especially considering Shamoun is Assyrian (not of African origins)! *
Sam Shamoun Admits to Leaving Christianity for the Racist NOI:
It appears Sam Shamoun has been unable to free himself of the shackles of discrimination and resentment. In his conversion back to Trinitarian Christianity he merely redirected his hatred, resentment and vilification away from a race of people but towards a religious group, Muslims (people of all races), and Islam. Sad :(
*NOTE: IT IS NOT CLEAR IF SHAMOUN STAYED WITH THE RACIST GROUP FOR 4 YEARS.
Video is found at the MuslimByChoice channel on YouTube, the video description:
The following excerpt was taken from the debate took place at the Center for Religious Debate, Michigan, USA between Farhan Qureshi and Sam Shamoun
The Topic was Is there Trinity in the Old Testament?
Click here to watch the full debate
Imam Shabir Ally on racism:
More on Sam Shamoun: