Because we have a very good series of arguments that what he did was wrong (jurisprudence).

The important thing isn't whether he was right, or wrong, its about the capacity to demonstrate why he was wrong to others. If someone can better present a case why someone wasn't wrong (or not the right person) then whether they are good or bad is irrelivent, as they'd be found not guilty.

Obviously its an extreme case, but if the defence proves Brady was innocent, even though he killed those kids, it doesn't matter whether killing kids is good or bad, it matters how you argue your case. Same way it doesn't matter if your intentions are good, you can still be adjudged as bad.

Typically, a jury in this case.

It isn't that Brady was bad, or that killing children is bad, but the capacity to demonstrate it one way or another.

At which point, its not about good or bad, its about how well you can convince others that something is good or bad.

Obviously I think he's bad, but the key is proving that, not what you think, or feel, but your capacity to influence others. Certainly, he convinced at least one person that sexual torture of children wasn't a bad thing to do (Hindley).

"One Nation Under God, has turned into One Nation Under the Influence of One Drug"
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In accordance with my leftie liberal soft view on crime I would have hanged the b*****d without trial and just because I felt like it.

Many times.

That is possibly not a contradiction. Perhaps on the balance of probability, a species wide or wider benefit could actually filter down an advantage to individuals that trumps advantage gained by more 'selfish' behaviour in certain circumstances.