He was the last seer in the world, so that may have frightened some. There were theories that he used his blood magic to control the minds of the great states, but I always thought those were ridiculous.
No, he was feared by some because of the old legends about him. That he was a necromancer, that he made pacts with the Fey, that he was a demon. Things of that nature.
I do not doubt that he used to be a horrible, awful man. No one could live as long as he did without doing some... less than moral things. He may very well have raised the dead with demonic powers the Fey gave him. There is no way for me to know for certain.
What I do know is that he refocused his efforts to good at some point along the line, and that is what matters.
-Jack manages to find a pub across the street-
-Yes-
-Somewhere that doesn't ask unpleasant questions-
What if there’s no better word than just not saying anything?

Because they would fear his ability to command Magic?