SEASON 4
"The Doomsday Sanction"
A scientist is murdered, beheaded (off screen; well, head possibly torn off).
"Double Date"
The parents of one person are murdered (not seen) in a flashback scene.
"Epilogue"
A future scene shows a villain dying.
I didn't see all of season four.
SEASON 5
"I Am Legion"
A member of the Blackhawks from the World War II two-parter seasons ago, shows up, only to tell us he's the last surviving member. He's quite old now, so there's no telling how each member died old age, battle, etc. but they're dead.

The New Number 2: "Are you going to run?"
Number 6: "Like blazes. First chance I get."

"Metamophosis: Part I"
A scientist died, but of old age or cancer (kind of a generic unspecified death, and off screen before the events took place).

The New Number 2: "Are you going to run?"
Number 6: "Like blazes. First chance I get."

I'm not exactly keeping track, but as I re-watch the series:
Somebody died in one of the episodes of the three-part pilot; an astronaut who never came back from Mars and was replaced by an alien.
The heads of three people who were murdered are found in Felix's place in "Paradise Lost: Part II". Felix also died.
In "War World: Part I" the dead bodies of defeated aliens are seen lying along side the pond, inclouding one that was murdered just off screen during a battle.
"Fury: Part II"
A flashback scene shows a small village mascred by invaders. No on-screen deaths, but people were shot dead in their homes and then the whole town was bombed by a jet fighter.
Also, a tanker captain dies when his heart gives out.

The New Number 2: "Are you going to run?"
Number 6: "Like blazes. First chance I get."

Tala, the Ultimen, several members of the Legion of Doom who were either killed in Grodd's failed mutiny or frozen by Killer Frost and unable to escape the ship's explosion, possibly Paran-Dul and Kragger, several Nazis aboard the airships the US Navy and then the Blackhawks destroy.
One of my favorites, though it was undone when Batman and GL reversed the timeline to before Chronos' left his garage (and Batman's nifty little program ensured that he
never
left) is Chucko. If I was a villain capable of time travel I think I'd want to dispatch of traitorous minions the same way. And Chucko's reaction as the asteroid approaches is great, as is Chronos' warning to the other Jokerz when he gets back. "Do you know what killed the dinosaurs?" "Uhhhno sir" "Well Chucko does"

Grodd. Desaad. Darkseid (though he returned). Stepenwolf. Dr. Destiney's ex. Shayera and Katar Hol's past selves. Felix Faust. Grundy. Orm.
Let's be bad guys.

Flash should have died when he was zipped out into space
Actually, space is not an instant death. Even in real life.
Brain damage, strong possibility. But death would take a while.
Let's be bad guys.

I can say this.Flash should have died when he was zipped out into space..I assume Grod died by this way also.