Good point bfisher! I was expecting a trinity when I first read it.
One of the things I liked about the temptation section was the way each tempter represented a deeper layer in Becket's soul. The fourth tempter uses spiritual pride--the most dangerous temptation of all as the Archbishop realises:
"The last temptation is The greatest treason:
To do the right deed for the wrong reason."
What should be service becomes selfishness regardless of the good that may be done:
"Servant of God has chance of greater sin
And sorrow, than the man who serves a king.
For those who serve the greater cause may make the cause serve them,
Still doing right:"