You would need steeper slopes in the gun states vs. the nongun states to make that point. Probably been done but I don't recall the gun states being steeper. Also, you need to look at it by a county by county basis to control for demographic differences.
NY would certainly contend the opposite is true. Murder plummeted when gun ownership was severely restricted. There argument also has little validity.
In the US violence is correlated closely to ethnicity, education, age, gender and income related.
For example African American males are murdered at rates 8-10X that of whites males and nearly always by other AA males. If it were possible to separate the statistics we would have a very European type murder rate (around 2.6) for non AA's in the US.