I prefer to think Barnabas killed those who deserved it, like Jason McGuire, or Nathan Forbes, but he did kill his share of innocents. I don't hold him responsible for anyone who died as a result of the curse--Sarah, Angelique, Naomi.
Pre-curse, Barnabas killed Jeremiah as a point of honor, which might seem stupid to us but was most important in those days. And again, Angelique put the spell on Jeremiah that caused him to fall in love with and marry Josette, so she was responsible for Jeremiah's death, and Josette's, too.
It was all just a nasty mess!
Love, Robin