Over here, tips are extras and often go into a pool to be shared between staff, it's illegal for them not to get minimum wage. Anyway, I though Don said he tipped x per cent, he just wasn't going over and above.

The timeline of this episode makes exactly zero sense.

It wasn't that 'everyone knew' Curtis had committed sixty murders, it was that he told Don that he had. And I'm sure Don believed him, and if you had twelve Dons in a courtroom then Curtis would end up back on death row.
I think there has to be a difference between legally right and morally right, otherwise how could you say that a particular law is wrong? The point is that even though Don knows that Curtis is a murderer, he hasn't been tried for his murders, and so the state shouldn't be executing him.