Things like this happen occasionally more or less everywhere.
Young men can be easy prey to extreme ideas - right or left, religious or secular. They normally end up destroying their own lives and causing heartbreak to their families and friends. It's not an excuse to be complacent- but we have been here before.
In the 1960s there were bomb attacks on the Temple of Peace, the then Welsh Office and Llanishen Tax Offices by the nationalist group MAC.
In 1973 a bomb exploded at the Portugese vice-consulate planted by home-grown anarchists.
In the 1980's Welsh republicans bombed the British Steel HQ in Gabalfa, Tory offices in Whitchurch and the Army Recruitment Office.
It happens from time to time - it's a Police issue not a reason for moral panic.
In passing, the UK government was ready to turn a blind eye to British Muslims going to fight in Libya and, until recently, Syria. In that sense we may be reaping what we have sown.