Controlling the monster is only half of his battle. Controlling the man is the other part - and Spike's own blessing and curse is that he's not thinking about his human potential for evil - only the demon's.
Now that is a *very* good point!

Re. William then I think he *is* (was) a good man, but - as you point out - this can be a two edged sword. Just look at Willow ("I'm very seldom naughty.").

And of course Spike isn't William, just like Angel isn't Liam. What I love most about Spike's arc in S5 is him trying to find out what sort of person he is.