What would you say to someone who believes, and I quote, "sometimes, they don't even need a trial. After all, you have more than enough proof from what you've seen and experienced."?

If this is regarding a "Justice System" then ask the person to empathize. How would they feel if the media coverage only showed things pointing towards guilt regarding their life?

Wouldn't they want ALL the evidence that supports a declaration of finding them  Guilty or Not Guilty to be carefully weighed in front of an impartial group of peers? This is where reciprocity takes a step forward . . . Wouldn't you want to put forth an great effort in the hopes that a great effort would be put forth for you?