idiomatic
/ˌɪdɪəˈmatɪk/ adjective
- using, containing, or denoting expressions that are natural to a native speaker. "He spoke fluent, idiomatic English"
(Some native speakers are not familiar with the term "idiomatic" as defined above, and will think it means "is an idiom or very commonly used expression with a non-obvious meaning, such as 'Cool as a cucumber'." "I know what you've done to some of them" is not such an idiom.)
Yes, "I know what you've done to some of them" is idiomatic English. It is slightly awkward, but it is the necessary construction to produce the intended meaning for the context.
It is slightly awkward because the noun phrase "some of them" is more commonly found at the beginning of sentences rather than at the end of them, but there is nothing preventing you from using this noun phrase like any other noun phrase in any location you can use a noun phrase.