In a lot of ways, it makes total sense that Artemis and Orion became such good buddies. She was the goddess of the hunt, and he was an awesome hunter. Both of them like nothing better than running around in the woods all day on the prowl for some fresh game. (Yay for friends.)
In some ways, though, Orion and Artemis were way different. See, Artemis was a sworn virgin, and took her vow of chastity super seriously. Orion on the other hand was a convicted sex offender (for raping princess Merope).
Given this major difference between them, it's not so surprising that in some versions of the story Artemis ends up killing her hunting buddy over sex-related issues. Check out the alternate endings in "The Less Short Story" for the details.
Click here for more on Orion's best hunting buddy.