I am bit lost by the きたんだろう in this sentence.
something like you do what you want with me? I read somewhere that this きたんだろう means something like the speaker "knows" what to do. but i am totally confused.
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Context is king! I'll try to answer your question, but trying to do it without any context is a pain in the neck.
だろう is the form of でしょう, which you use when speaking with a person you're familiar with and you can afford to use it.
ん is a contraction of の.
These two forms original, dictionary form would be んだ, alternatively のだ or のです. でしょう (or だろう) when linked with の/ん forms sort of rhetorical question. The speaker seeks agreement, he's not really asking the question.
te-form + きた indicates that an action has been taking place over some period of time.
勝手にする means "to do something selfishly".
With these 4 points in mind, I believe the right answer to your question would be something along the lines of "I guess, you've been doing that without any regard to anyone or anything, haven't you?!"