Like, the Xel'Naga were all worried about creating their .. successors, as eventually even the Xel'Naga die ... so they can combine to recreate the theoretical "Xel'Naga"
Amon is a Xel'Naga trying to avoid death, while at the same time killing his brothers so he can reign supreme (think about it you're not really all powerful if there's a bunch of other Xel'Naga just as powerful as you running around)
Abathur states that the Primal Zerg don't die of Old Age right ? And that Amon envied and wanted their ability to seal essence, thus making him like the Primal Zerg, thus allowing him to live forever, while his Xel'Naga brothers... even though they were long lived, died of old age and Amon hijacked Overmind to kill them off faster. And put the Overmind on a crash course with the Protoss to make sure the "Xel'Naga" could never be recreated to oppose him in his rule.