Your premise is mistaken. Sri Lanka's murder rate is lower than that in the US, but that doesn't make it low by international standards. I doubt that Buddhism has much to do with it. For example, both Israel and Jordan have much lower murder rates.
The premise is that Sri Lanka has unusually low crime and unusually high suicide, and the following argument is trying to explain that. But judged by murder rate (the most consistent crime stat), Sri Lanka doesn't have unusually low crime.