I used you in the sense "I knew that letter was for my sister but I pretended I didn't know so that I could have a chance with you since I like you". I don't think it implies anything else
Yeah, then you’d be taking advantage of them, being in a better position for personal gain. You use someone for gain, implies availing oneself of something as a means or instrument to an end. which has a negative connotation
I don't think it has such a negative connotation, he just wanted to get closer to him. Surely he did it in the wrong way, lying is never the best option, but he didn't mean to hurt him
That’s why “used” is not the appropriate
I wonder if “I used you” is the correct translation, since that implies he’s trying to get something from him. It would make more sense if it were “I took advantage of you” as in used his timidity to try and date him.