I pray thee, mark me.
I, thus neglecting worldly ends, all dedicated
To closeness and the bettering of my mind
With that which, but by being so retired,
O'erprized all popular rate, in my false brother
Awaked an evil nature, and my trust,
Like a good parent, did beget of him
A falsehood in its contrary as great
As my trust was, which had indeed no limit,
A confidence sans bound. (1.2.108-117)
Prospero suggests that Antonio's taste of power awakened in him an even bigger desire for power. Prospero's loyalty to his brother was so great, and his trust so complete, that he really didn't see this coming. That, of course, allowed Antonio to take it farther.