Why would he want a rampage ? Why would imprisonment time would make Amakthel mad ? he's supposed to be a being beyond those concept of space and time.
The creatures he created are just there to help him gather his power back, they're not going out of their way to attack the population, they're found in place of high power, trying to gather it for their master to help his awakening.
Additionally, there's something to take in account : Amakthel was unchallenged, there was none fighting him, simply due to his power being beyond all his kind as the Sher'thul mentionned.
So Amakthel wasn't waging any wars due to the lack of enemy and was then spending his time in creating things like he did for the Maj'eyal world and sun.
Additionally, from the excavations of the dwarves related in the Mouth lore, we learned that Amakthel place of power was actually on Majeyal, as the excavation even ran into his golden throne .
So basically his presence was a guarantee of peace, due to the previously mentionned lack of fighting and warring surrounding Amakthel, his rule was simply absolute and none of his kind could depose him as none had the kind of power required to face him.
And i think here's the trick and the actual real nasty one : Arawe
But the Sher'Tul were unhappy with their position in the world. Being made with the same passions of Amakthel they had no desire to be simple pawns in the wars of gods. And so they resolved that they would destroy the gods, and become masters of the world themselves. In this they recruited the help of the other races, whom they pitied, and also enlisted the aid of Arawe, Lady of the Waters. Arawe, unlike the other gods, took no part in all the battles and wars and made no creations of her own beyond fish and other sea animals. She agreed that she would aid the Sher'Tul in stopping the other gods, and that she herself would retreat to the farthest depths of the seas, never to interfere in the affairs of mortals. Aye, and it is said that the oceans have since doubled in size, with the tears of Arawe's regret.
Arawe basically gave to the Sher'thul those weapons so they could go and kill her kind. That in itself is odd, as Arawe being a godling she couldn't ignore the inner nature of those Sher'thul, she knew they were inside of them basic power hungry warmongers.
So she went out of her ways and decided to help them in destroying her kind by forging godly weapons, and promised the sher'thul that she would never take part of anything on the world and would go in the depth of the oceans forever
"it is said that the oceans have since doubled in size, with the tears of Arawe's regret"
Yeah, i'll believe that ... except that it is clear Arawe was a traitor to her kind, and the Sher'thul were supposed to trust someone that can betray her kin that easily ?
No, in my opinion Arawe knew perfectly the nature of the Sher'thul , and she pushed all the right buttons to launch them into a war against her kind, and forged weapons capable of doing the impossible.
Once they even managed to depose Amakthel, Arawe knew that it was only a matter of time that the Sher'thul would then , out of enemies (she even convenientely went hiding away), turn against each other.
And that's exactly what they did, so what is her status now :
- Amakthel is not the absolute ruler anymore, the place is actually free
- Her kind is no more, so she will have no power that will be able to put her in danger once she rules over everything
- the Sher'thul are dead as she planned, by their own hands
All those thousands of years waiting hidden in the depths of the oceans , away from every eyes and concern are getting to an end.
But why didn't she do her move :
-Gerlyk is still around, even if he moved beyond the Void, there's still the possibility some couple of sorcerer may open the portal, and from the lore he's not a bad guy, Arawe is then waiting for some idiots to come kill the sorcerers and close the portal to the void forever.
-Ralkur is still around, though while this one is a nasty, he's not going to come back as he's fleeing into the infinite dungeon so that's one that isn't a danger for her and neither is the Sher'thul following him.
-Amakthel, the Sher'thul despite all the godly weapons and the dismembrement were still unable to destroy him completely, he's even trying to awaken by sending odd creatures gather power. So for now Arawe waits for an idiot to go kill one of the last Sher'thul that is trying to help Amakthel coming back
-There's a Sher'thul still alive in a fortress , somewhere, and he has one of the godly weapon, Arawe certainly wait that someone get to him and kill that one , unless that will be the one that will try to help Amakthel
So while the attention is elsewhere, she waits