You know another way to look at it is that your BIL was simply looking for someone or some group to acknowledge and accept him, give him a sense of purpose, and a sense of hope for him to turn his life around. In other words, deep down inside he wanted to be a good person. This could've been accomplished outside of a religion but it just so happens that it was the Watchtower religion that gave him the validation that he needed.
And I think you're absolutely right. The Watchtower was a good thing for him. It provided him with the moral compass that he otherwise lacked and turned his life around. The Watchtower isn't totally evil, after all. Just mostly evil.
But in some cases, if we believe in the wrong thing, our personal conscience may still lead us astray - in that we won't know we're doing something wrong, so we won't feel that twinge of guilt or warning.
It still goes to what constitues the wrong thing. The bible, for example, defines many things that are wrong and then prescribes punishments accordingly. From my perspective, many of those wrongs are victimless crimes for which some of the most horrendous punishments were meted out under the understanding that they were mandated by God himself. How do you tell if something is wrong if not by the golden rule? Being the film buff I am I am reminded about that line in "K-Pax" when Kevin Spacey says, "every creature in the universe knows the difference between right and wrong."