I think you're blowing this all out of proportion.
Since you say he hasn't shown any aggressive behavior towards you and you say that he uses them as a tool, you have no need to worry. Knives do not kill people, people kill people. That is why you avoid people who do bad things or you suspect do bad things. If you feel you have nothing to worry about him, then you have nothing to worry about his knife.
A knife is indeed a tool. I use knives everyday at my job and around the house. Since the dawn of man the knife has been one of humankinds most reliable tools. We have used it for hunting, food prep, cutting string/rope/chord, modification of objects and the list goes on. Killing is only one use. You have a car. You can kill a lot of people with that car. I can ram my car into a group of school children when I commute to work tomorrow. But I choose not to because I am a rational human being.
A collection of 20? Pft I have a collection of 50 knives, each of a high quality and sharper than a razor.
You have nothing to worry about if your only concern is the knife and not him. If you must, talk to him about it. You are his girl after all, and you should tell him when you don't feel right about something. Just tell him how you feel.
You can't be serious. "Lose the knife collection or lose you."
Unless he is putting the collection above her in priority, you honestly cannot say that. In fact, saying that is part of an unhealthy relationship. You DO NOT tell people to simply give things up all because you don't believe in it. You compromise. That is terrible advice.