Learn Swedish: Han vet att rädsla inte skapar fred. - He knows that fear does not create peace.

Because they are abstract nouns being used in a general sense. Just like in English, when you make a general statement about concepts like fear (rädsla) or peace (fred), Swedish leaves out the article completely. If you were talking about a specific fear, you would use the definite form (rädslan).

Why does inte come before the verb skapar in this sentence?

Because att rädsla inte skapar fred is a subordinate clause (a secondary clause attached to the main clause by att). In Swedish subordinate clauses, adverbs like inte must be placed before the verb. You might know this as the BIFF rule: Bisyats (subordinate clause), Inte Före Finita verbet (inte before the verb).

I often confuse vet and känner. Why is it vet in this sentence?

You use vet because it is about knowing a fact or a piece of information (knowing that something is true). You only use känner when you are talking about knowing or being familiar with a person, a place, or experiencing a physical feeling.