SPOILER!!! I think everyone is missing something
I have read several posts on this movie and I must say that I haven't seen a single one who isn't misinformed about the identity of the Ripper. Contrary to popular belief, it is NOT the guy with the sick little sister as everyone would seem to believe due to a long explanation sequence....no, this is mainly missed due to its subtlety (in a Fulci film?!?)in comparison to the more overt explanation. The fact of the matter is that the professor is truly the Ripper. The duck voice is simply another tool used by him to misdirect the police (and the audience) so that he can get away with the horrible crimes he has committed.
Think about it: there is a scene which has no other relevance to the script but to reveal this....when the professor is playing chess or checkers or whatever with the computer, he says to it, "You think in fixed patterns. Just like a cop." Why else would he say this? Granted he's arrogant, but does the duck voice not also continually mock the detective's intelligence? Soon after, the final confrontation and explanation are presented, and once again....the very last shot of the film is a freeze on the professor as he walks away from the detective....why else would it end this way? Fulci is telling us that the Ripper has just duped the police and got away scot-free. I must say that due to these smart twists and the subtlety in their presentation (not to mention the most brutal gore to be found in Fulci's entire canon) this is my hands-down favorite of his films....excellent example of the giallo as well...
I'm sure many won't believe me, but watch it again....it's fairly obvious.