He didn't! Recent scholarship has instead emphasised how successful he was and how his depiction in later sources is sorely mistaken. Rather, it has been convincingly argued that his move to Sicily was a temporary one, to secure an important province and to defend North Africa; both provinces were essential to the finances of the state and under threat from the Arabs, so it was of utmost importance to defend them. As for his assassination, it can most plausibly be associated with the Siege of Constantinople in 668 (you might have read about the 'siege' in the 670s, but this has been definitively redated to the end of Constans' reign). From the sources, we don't get the impression that Constans did anything to help defend his capital, such as by withdrawing troops from the west, which I think is a far more likely reason for his murder in Syracuse. I have written more on Constans here and cited some books describing this view. I can also recommend a very recent book, J. Haldon, The Empire That Would Not Die: The Paradox of Eastern Roman Survival, 640–740 (2016), which tells you everything you need to know about this period, though it is very academic in tone. Peter Sarris' book from the link is much better as an introduction I think:

Constans II was to be remembered in the Greek and Latin sources as amongst the most reviled of Byzantine emperors. The reasons for this are clear: these accounts were written by hard-line Chalcedonian churchmen who regarded his treatment of Pope Martin and the monk Maximus with horror. Theophanes, for example, claimed that the Emperor ‘was hated by the people of Byzantium, particularly because he had brought ignominiously to Constantinople Martin, the most holy Pope of Rome and exiled him to the Klimata of Cherson, because he had cut off the tongue and hand of the most learned confessor Maximus, and had condemned many of the orthodox to torture, banishment, and confiscation of property.… For these reasons he was greatly hated by all.’ None of the Emperor’s actions could be praised; each and every one had to reveal his baseness of character. Constans’ decision to head west to Italy and Syracuse, for example, was not an act of daring aimed at saving the empire and freeing the Christians of the east from their Arabian captivity: rather, ‘it was out of fright’ for ‘he intended to transfer the seat of the empire to Rome’. It is because of such bias that our Miaphysite Transcaucasian sources, such as the Armenian History, are so vital. Yet even the prejudices and distortions of Theophanes cannot entirely obliterate the traces of what was clearly a reign of breathtaking creativity as well as extraordinary courage and imagination.