How long does the knife need to be before you'd need to question the person's intentions? How many spikes on that handle? How deep do the serrations need to be before they cause concern? Would you like to ban those out right or provide licenses to make sure the person carrying the knife is trustworthy, competent and trained?
Who would you give the authority to question that person's intentions? What criteria would you use to judge that intent? How would you penalize someone who doesn't meet your criteria? If it's an outright ban what is the punishment? If not a ban where and when could a person carry one?
Jack over there thinks all knives should be banned so he'll use your fears and add his fears to pass stricter laws than you intended but they're all for the good of everyone right?
I don't believe in enforcing a ban on knives but I can honestly say if a knife looks as though it has no practical use other than to kill then as a police officer I would confiscate it and penalize the owner for carrying it. And if there is a criteria as you say it would certainly be a history of violence on that person's background check.
If I was a police officer and I searched someone and found a silencer on their personal side arm I would have to question their intentions regardless of their demeanor or how they smelled.
I don't believe in banning them at all I'm just erring on the side of caution.
Sorry if my opinion upsets you.