There are an estimated four hundred British fighters currently fighting in Syria. The question as to how the Muthanas of this world join designated terrorist groups like ISIS is more complicated than the media would like you to think. The story of Aseel and Nasser Muthana illustrates this point well.
The differences between Sunni and Shia Islam are political in origin, and remain so to this day. The way that Muslims are positioned throughout the world (70% Sunni, and 20% Shia) has heavily informed the politics of various countries and created much geopolitical tension and associated extremism between the two sects.