Personally I believe that it is more of the former than the latter.
All religions have taboos and ways of doing things, if you break those taboos or go against the grain in doing things you usually will be considered to break the will of the divinity that is being worshipped.
I doubt that the Sani Sabik would go around waving pitchforks and torches if someone refused to take an opportunity to seize more power when possible they would seek to seize it for themselves instead, there is probably a very strong survival of the fittest thing going on in their circles on many different levels which makes the machinations of the Holders seem mild in comparison.
Of course as a cult it is quite varied and probably littered with different personality cults that are more or less shortlived.
Just look at the Blood Raiders, a personality cult based on the Sani Sabik dogma.
There are several differences between the Blood Raiders and the mainstream Amarr religion.
They believe that the blood of clones is of the purest kind, therefore removing the aspect of Amarrian religion for Amarrians from the whole thing, opening it up for all the different bloodlines.
Their views on slavery are quite a lot harsher than for the mainstream religion, basically using slaves as a workforce and a blood farm to satisfy their religious need for blood.
Since the need for blood is so essential in their cult, they have replaced a lot of the spiritual aspects of the Amarrian religion with just the act of acquiring blood and sacrificing it to their God. When you have such a central and overriding aspect of a religion then basically for them the only Sin would be inability to acquire more blood to their God.
Nothing else matters.
Only blood.
Of course you have the fundies that spend their days hunting for more and more blood to the altar (and bathing) and the regular folk that go to the temple on sundays to spill blood on the altar which they buy from the vending machine in the front.
So as you might notice, religion is filled with so many different aspects and personal nuances so that you could spend your time talking about it until the cows come home.
I'm not sure it's true that the Sani Sabik don't believe in sin.
I think where the main difference between Sani Sabik is the belief that some people are selected
savants (i.e. supermen), and everyone else is destined to serve them. Even a person of low birth, such as a commoner, could potentially be one of these savants.
On the other hand, the Amarrians believe that greatness can only be passed down through one's lineage/ heritage.
So if Orthodox Amarrian was compared to Roman Catholicism, then Sani Sabik would be like Protestantism. Instead of a Catholic priest reading to his audience in Latin from a chained book, Martin Luther would be telling everyone they can own and read their own Gutenberg Bibles in German.
The Sani Sabik religion started out as a moderate departure, and then it mutated and just became weirder and weirder.
There are thousands of different sects of Sani Sabik, from the mild Gallente social circle of sharing your own blood voluntarily with your own friends, to the Blood Raider's extreme use of force and kidnapping to extract blood from others.
I think that there is a hedonistic sect of the Blood Raiders, and they probably believe that their sins are being washed away, but some BR's such as Naupilius see Sani Sabik and the BRs as merely a means of reinforcing of the lost elements of Orthodox Amarrian faith.
For me, the danger Sani Sabik posed to the Empire wasn't the use of blood in their rituals, it was because Sani Sabik would undermine the social order of Amarr Feudalism. Now, EVERY man (or woman) could potentially be ruler.