I finished the game, read wikipedia but I have still problems to understand the story.
Main problem is the timeline, because we see many thing from the past but I am not sure about the order of things.

So I try to understand the timeline:
- The demon (final boss) posesses the necromancer
- The necromancer kills the keeper of balance, the circle goes insane
- The necromancer raises Kain as vampire. The demon wants to kill the circle to ruin the world. No idea why he raises a vampire to do it and does not kill the other members himself.
- Kain meets the oracle who is actually the time guy. He shows Kain a way that would lead to the destruction of all vampires.
- Kain kills a good king in the past who was manipulated by the time guy.
- Because the king was killed by a vampire a crusade against vampires starts.
- The other vampire kills Malek to end the crusade (see intro). Why does the necromancer raise Malek to protect the circle members (intro) and later I (also raised by necromancer) must kill him?
- The crusade kills all vampires except me, I kill the time guy.
- In the end it is revealed that the necromancer was posessed by a demon who wants to destroy the circle and lead the world to ruin.


Some things are wrong in my timeline. Well things are hard to understand when the time of events is not shown (except going 50 years back in time to kill good king) and time travel is involved.



PS:
Ages ago I tried to play soul reaver, but controls and camera made me quit. It was hard to walk in a straight line and see the enemies I fight. When a remake was anounced I started playing blood omen to see the whole story in the right order. I hope camera and control are better in the remake.

You ask something that fannot be easily answered. As you did get such questions - every players does. Rumors and assumptions are Legacy of Kain series plot. Not officially answered. Sad but true.
Maybe better to say (or maybe not) that despite seems finished story it wasnt finished in first title and original story writer suppose for continuation.

My thoughts. I think there is conflict in Kains birthdate (and he was born as Keeper of Balance). Necromancer was not possessed anymore when he hire assassin's and rise Kain, do he? Or that recon from later titles? In next titles necromancer shown are kinda ally, hostile only to he killed realize thats required to restore balance. He mean to make himself died.
Or it still was in original BO... Necromancer shown as one real balance restorer, who try to fix everything (letting himself died at the end too).
Same role he supposed for Kain. Do the dirt job and kill himself. But like how Necromancer need someone to kill him, necromancer forget to hire someone to kill Kain (and our main hero become too powerful to do that).

Also. As Keeper Kain was cursed from born too. All keepers mean all keepers.

There is info SK planned second titles, i didnt find that on fansites but only on OGDB. Seems not accjrate, game was took actually before some plans can made for future titles. Not sure how accurate that information. For what we got there is like nothing explanations in the plot of 4 titles, SR1-2-Defiance are only perfectly connected trilogy. BO2 alternate timeline entirely. Nosgoth (online game) have more canon plot than BO2. Qnd they all not like really connected to BO1 (has another writer).

So there is variations was Kain jealous to Raziel or it was trick to rise new keeper of balance (and was Raziel born as such one?). If so. Why Kain and necromancer has such bright moments of sane been insanity corrupted-cursed? not explained.
No answer given by any titles. Many thing are up to fans assumptions.

Blood Omen concept plot and most of development procces made by one company, ended by another. As you know. Something was cut. The realm and whole game actually too "small" and just repetitive (even aside of 101% completion) with time travels and such.

Trying to understand probably a no go way, especially when you come to next titles cause of new lead writer. Game hvae unlikied faith. Some original creators able to make 1 and second title and then it goes to new writer or to new company entirely. Blood Omen just like Diablo never get such first title as original game, they got altered during dev process as acclaimed by another company.
Seems no one mean to make plot self-explained at Defiance moment, there was cancelled titles... You should know that series are not ended. Just not. Like Half-Life 2 episodes. And i assume no one care there is issues in plot at time when Blood Omen released.

I suppose there not and not possible to explain issues in plot. We can only assume something. Like Kain maybe wasnt keeper of balance at all (but that fome after SR and Raziel version of such keeper). Maybe he end to be keeper at his death*. But Raziel if keeper he died twice. As seraphim paladin, as vampire. When that born of the keeper start and ended? Maybe im wrong who was/not keeper here. Kain was, Raziel wasnt - as default (but not official) version (and there is no official versions, really not).

The only what seems more obvious. Nosgoth wasnt humans realm in the first place, and not human-based creatures was fiest keepers but vampires-based. And to restore balance it meant to human* circle got cease to exist and replaced by vampires. So did that demon was villain is another open question.

Old forums are better place to discuss that. Closed rorums, yep. Including Square-Enix-EU forums. I guess no one provide good answers to your question here (and im not) or no answers at all.
So. Maybe Fandom Wiki? Survived ones fan-sites? Attempts to stucturised the plot was made there and more at forums for variations. Some places for you to go still exist and they are better to ask plot questions than here.

Thanks

My thoughts:

The whole chaos starts when the necromancer kills the keeper of balance.
At the end of the game we see that the necromancer is posessed by the demon.
So I assumed the necromancer was posessed by the demon all the time.

No idea why the necromancer acts good sometimes.
Good as in "against the interests of the demon", not as in good. We talk about a necromancer.

Theory:
The demon wants to first corrupt, then kill all circle members. So he posesses the necromancer.
He kills the keeper of balance to make the rest of the circle go nuts. But the necromancer cannot kill himself for whatever reason. So he needs to create a creature who can destroy the circle, including himself. Thats the protagonist. But first he has to help me (starting by killing me and raising me as vampire) so I survive long enough to become strong enough to kill him.
The time guy does not know that the necromancer is posessed. He manipulates me so that all vampires (including me if his plan works) are killed in order to save himself.

This theory has (at least) one problem: Why does the necromancer raise Malek to protect the circle?

Maybe the demon wants to end all life: the circle, humans and vampires?
If the demon beats me in the final battle, all powerful beings in this world except the demon are gone.