| Maybe the way he did was not okay, but if you look at good vs evil, he did his job..
When he saw them he may could have warned them first "Stop you lousy slavers" and if they stopped maybe he could apprehend them, if they didn't stop he could stop them the hard way (what he did with running them over)
When good aligned, it is not wrong to kill, it's just how you justify it..
Killing a bunch of gnolls because you think they are merciless SOB's who do slave trade is evil.
Kiling a bunch of gnolls because you know they are merciless SOB's who do slave trade is good.
(This all comes from a perspective in the D&D world, not reall world good vs evil) | 
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