Learn Russian: Преступник угрожает свидетелю.

The verb угрожать (to threaten) requires the Dative case for the person receiving the threat, without any prepositions. While in English we threaten a witness (using a direct object), Russian treats it as a communication or action directed to someone, similar to handing a threat to them.

Could I use the preposition к (to/towards) before свидетелю here?

No, you shouldn't use a preposition. Verbs of communication and hindrance like угрожать (to threaten), мешать (to bother/hinder), and помогать (to help) govern the Dative case directly. Adding к here would be a grammatical mistake.

Why does the Dative form свидетелю end in -ю and not -у?

The dictionary form of the word for witness is свидетель. Because it is a masculine noun ending in a soft sign (-ь), it has a soft stem. Therefore, it takes the soft Dative ending -ю instead of the standard hard ending -у (which you see in words like брату or студенту).

I know the word for crime is преступление. Are преступник and преступление related?

Exactly! Both words share a root that literally means to step across or transgress (crossing the boundary of the law). The ending -ник is a very common suffix used to describe a person characterized by a certain trait or action, making преступник the person who commits the crime.