I guess he didn't want to be 'toothless' as his brethren, and maybe, some action? [we see in Raiden's story from MK4 to MK Deadly Alliance that he couldn't do anything to help Earthrealm as long as he was an Elder God. He had to relinquish his status to help fight Quan Chi and Shang Tsung, even though he was a lot less powerful than as an Elder God.]
Anyways, when the realms were created, each had one defining source of 'power', Seido having order, the realm of chaos having, well, chaos, and Earthrealm having...frankly, I don't know, but that's what Shinnok saw that would give him immense power and, as you said, worship. But to be the Ruler of Earthrealm, he had to relinquish his status [pattern?] and, thus, a lot of his power. So he forged his amulet to retain most of his powers whilst now being able to enter the realms.
Raiden saw him as a great threat [rightfully so] and went on an all-out war against him and, perhaps, his forces? [some lesser beings made by Shinnok or that just decided to follow his lead, that's unclear even in the original timeline.] and, assuming Shinnok had some sort of army, that could mean Raiden's forces were the Saurians, Reptile's race [That can explain how they were decimated] and Raiden won, banishing Shinnok to the NetherRealm immediately afterwards. There, he was tortured by Lucifer himself until he joined forces with Quan Chi and overthrew Lucifer, yadda yadda.
Well, to me the Elder Gods [save Shinnok] are all lazy and selfish, so they do not interfere in the living of the realms, they're just worried about preventing Armageddon and the One Being's return. So what Shinnok did could be considered an act of rebellion and, thus, treachery. Elder Gods are weird to understand, dude.
And that last bit about the saurians esacping to Outworld...ehh, I've never heard about that. I think some old comics explain that [some, very few of them] Saurians escaped to Zaterra from Shinnok and Raiden's bloody war, and then Zaterra was taken over and merged with Outworld by losing 10 straight Mortal Kombat tournaments. But if the ruler of Outworld was Shao Kahn or the Dragon King Onaga, I don't know. [probably Shao Kahn tho, that mofo loved him some merged realms.]
Well, I hope that cleared things a bit. I had to answer this, Shinnok's ma fave!
Off-Topic: I believe Shinnok's MKX variations will be indicated by his gems' color. My profile pic [even tho I edited it] had green gems and other media shows him having red gems. Maybe the last one will have purple gems, though blue gems would look awesome.
I feel so alone, gonna end up a Big ole pile of them bones.
