That Time I Got Reincarnated as a Slime: Tensei shitara Slime datta Ken

[DarthEnderX]

Rimuru needed to kill a lot to evolve to a demon lord too. It was also a selfish and evil decision.

Except Rimuru didn't actually care about being a demon lord.
He only did it because becoming one would allow him to resurrect his murdered people. Which is not a selfish and evil decision.

Paradies had ZERO power UNTIL THE VERY MOMENT EREN MIRACULOUSLY ATTAINED POWER.

And how is that any different from, say, the goblins, before Rimuru miraculously showed up?

except now Eren wouldn't exist anymore, because Eren would be dead

Except Eren could pass his power on to anyone he wants before he dies.

And Eren didn't undo anything to commit genocide, he wanted to free his people of eternal oppression and threat.

Then Rimuru didn't do anything to kill that army. He wanted to free his people of threat and revive those already murdered.