I think we are still waiting on the far older Chinese civilization to catch up with that level of humanity and justice.
But, to go further, there's a period in Roman history called "The Social Wars". For the most part Rome rose to ascendancy on the Italian peninsula by "merging" with defeated states ~ I don't think even the blindest China apologist would argue any such thing happened in China during the Period of Warring States.
As a brief, I pretty much follow the sort of time-line and opinions about events expressed in this website: http://www.san.beck.org/3-13-Summary.html#2 ~ which is a very, very common timeline.
The Qin emperor was definitely into genocide ~ both BEFORE he became the first emperior, and after he formed his empire.
This is the one period in ancient times that has a death toll due to war anywhere near that regularly experienced in the twentieth century.
Then explain the thousands of different dialects still being spoken in China, if you think that the Qin emperor decides to kill off all of his opponents? The fact is, he was brutal, and there were mass murders but no less than any other civilization at the time (Romans, etc). And to say he simply wipe out the other 6 countries' population is ludicrous. If that were the case, then there is no one in CHina that would speak anything other than Putonghua, and there isn't, there are thousands of dialects.