I know this isn't proof of any kind, but to help illustrate how evil can INFLUENCE but not VIOLATE Free Will, read/watch something by a solid Catholic like Tolkien.
The One Ring doesn't violate Free Will, but it has a strong influence on most peoples. You don't "mess" with Sauron or get into a battle of wills with such a strong powerful being. That's how you end up his slave.
Likewise, Catholic teaching says that you don't start talking with the devil. Or, like Saruman (another bad guy in LoTR) he will use his powers of speech to convince you. While he speaks, everything he utters sounds eminently reasonable and good. Anyone who contradicts these words seems like an enemy and a fool. You should read how Tolkien describes his bewitching words -- it's brilliant.
Indeed. Look what happened to Pippin looked into the Palantir of Orthanc.