I, personally, didn't say anything about families of victims, nor would I - they're entitled to feel however they feel. However, I think people tag people like the woman described as 'courageous' and not people who would like the accused executed because her reaction is generally considered to be a response that goes against what most consider a fairly universal, innate desire.
Most people - as has been shown through bad responses in political debates over the decades, heh - would, if their family member was a victim, want to see someone punished as severely as possible, strung up, tortured, whatever. but the question is whether they'd choose, if their choice, to act on it Therefore, people tend to assume that the woman has, or has had, the 'normal' reaction. That she then chooses to espouse her beliefs instead of going with what she feels - that's where people get courage. Not just from WHAT she's saying but how she got to saying it, if you see what I mean. Which puts the family members on the opposite side in the 'natural reaction' category, not weak or bad. As well, they can simply be in favour of it in a general sense, though some are not. I'm not saying I agree with her being labelled courageous or not, just trying to explain how I think people who say it got there.
I think 'innocent people have likely been executed and given the statistics surely seem likely to be' is a valid argument against the death penalty in a general sense. It's not as if, barring some terrible Tom Cruise film advances in technology and human interaction, we'll ever be able to be 100% sure. Hence 'against it because it doesn't work.' I get your objection but it's like saying to someone who doesn't want to get on a motorcycle because they're dangerous that they're wrong about the motorcycle being dangerous, it's human error that's dangerous. Well, great, but getting on the motorcycle is still statistically dangerous and that's hardly gonna change in the forseeable future.
Plenty of people also have a variety of reasons they hold. People can simultaneously know that it's not a deterent, believe that it's unjust, believe that it simply shouldn't be used, believe that it's unconstitutional, believe that it's applied in a racist manner AND believe that it's too subject to mistaks. It's not like people are limited to just one specific idea.