Snape feared Albert Anderson from the bottom of his heart.
Not only because Albert has the power of the mysterious man, but also because he can do whatever it takes to achieve his goals.
It's a lot like a Slytherin, if not more than a Slytherin, because that guy can always disguise himself so well that he can fool everyone with ease.
This can be seen from Albert's easy use of the Imperius Curse to control Yaxley.
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