Grayson reverse kidnapped Dane. Holding someone in a place against their will is considered kidnapping. So they are BOTH wrong.
So why break Grayson's legs? Dane can hold a conversation as a normal person. He not only knows he did something wrong but also feels bad about it.
Grayson on the other hand... He barely even knows what feeling bad means. He can't really be reasoned with. He thinks there's nothing wrong with his actions.
If they slept together under these circonstances like Grayson is trying to force Dane, that would count as coersion to have sex so... Rape.
Anyway, I don't think any of it will happen that way
You can see Dane has been beating Grayson for the last few chaps and the guy was laughing at Dane' face. Beaking his leg wont civilize him, it will only shows how desperate Dane is for a way out, which is what Grayson wants to push Dane to confirm he is an omega. When you are 6ft underground, threating to break a leg or kill the only one who may know the way out is a stupid move. And spoiler...
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Dane will rape Grayson in the next few chapter, pushing him in the wrong even more

...may I suggest breaking his legs? That type of pain might be a good motivator to go back into civilisation.