In Justice League episode about the Justice Lords...

In Justice League episode about the Justice Lords, they say that the even that triggered Superman to become a totalitarian dictator was the Flash's death.
Why is that?
I mostly watch the cartoons and don't have that much extensive knowledge of the character's history but why exactly did the Flash's death had such a huge impact on Superman for him to basically drop his no killing rule?

Flash was seen as the “heart” of the League, the guy who could pull everyone together and keep everyone grounded despite their differences.

he was the most human of them all, everyone else was either an alien or some variation of a sociopath

Because Flash, both Wally and Barry, are just that likable. They're the ideal superhero.

DCAU Superman has a slightly shorter temper than other iterations of Superman, and he takes what happens to his friends very seriously, especially after the death of Dan Turpin. Justice Lord Superman apparently became friends with their version of Flash rather easily, and Flash's death at the hands of their Luthor pushed him over the edge.
Hell, in Superman: The Animated Series, another alternate dimension version of Superman was tricked into going full dictator mode after that world's version of Lois Lane got killed.

The fact that Lex Luthor orchestrated it.

>slightly shorter temper

This - except I'd call a bit more than 'slightly' - he's OOC in other ways, that were strictly to deal with the cost and expense, as well as the story-telling limitations they devised for themselves in STAS and them in moving towards JL/JLU.

In some of the BTS material they prepared for the DVD releases, where the go back and talk about the first season of JL, they admit that the biggest question they got asked at Cons and in interviews was about how easily it was for aliens/Brainiac/Villains, etc. to get ahead/beat the heavy hitters of the League, such as Clark and Diana, Wally and Jonn, etc. They specifically remarked again about Superman and how they regretted what they did which was a carry over from STAS (and they also talked about that in the BTS of STAS), such as needing the breathing devices in space and the space ship for interplanetary travel even within our solar system, etc.

>most human of them all

I think Jonn in the JL run fits this much better, especially given how they started Wally off at first half of S1.

What was wrong with Supes having a breathing device in space? Even if he's "can breath in hard vacuum space because comics" they could have made the point that he'd have to have the air to _talk_ as well, or that it was just more comfortable.

Flash was the "kid" of the cartoon JL (at least the original roster before they got Captain Marvel). Killing him represents the death of innocence.

Also, because you try making this dorky almost-teenaged ginger into an imposing evil dictator. He had to go or no one could take the episode seriously.

>We'll never get a Flash who's gone full dictator.

>The world covered in a red blur, crime being stopped before it could start.

>People's rights being trampled on, as Flash deems their planning to be 'too slow', but inevitable.