People who participate in something are often best positioned to criticise it, because they:

A) know how the sausage is made, and

B) stand to actually lose something if people believe them, i.e. they are standing by their point in spite of the negative consequences to themselves.

People on the dole who argue against (details / implementation of) social security should be taken seriously. Rich people arguing against tax breaks for the wealthy. Programmers against big tech firms. Etc.

So only murderers should judge other murderers?

Hmm.

technically the whole criminal judicial system is a licensed murderer

That's not at all how argumentation works :-)

https://news.ycombinator.com/item?id=18886753

'Actions speak louder than words.' ;-)

That's exactly how argumentation works...

You’re confusing argumentation with perception.