Yeah, Shinra were a weapons manufacturer who stumbled upon Mako energy in their weapon research and realised it was more profitable. I may be wrong, but I think they were trying to manufacture materia and that's how they found out that Mako could be used as energy.

I don't think it's too hard to believe that Sephiroth was just plain angry. He had basically been used as a tool by Shinra his entire life, and finding out that he had been manufactured like a tool would irritate most people.

There are also character traits within him that make him susceptible to megalomania and general insanity. Before his origins are discovered, he tells Cloud that he has always felt different, which is a modest way of saying special. He already believed he was superior before he found out that this was a genetic fact.

He then reads and believes that he comes from a superior form of humanity, which sacrificed itself for humanity. So, he believes he is superior, that his people are dead because of humans, he is already unhappy with the Shinra situation, so he decides to destroy everything that is inferior to him.

Once he had reached that stage, it doesn't take much to want to destroy people because of some ill-founded belief of ancestry with just wanting to destroy the world to gain power.

You could also argue that, because he was basically the child of Jenova, there was a genetic instinct for him to destroy.

Anyway, to sum up my long post, Sephiroth was a) vengeful and b) felt superior. These two traits of his character do not change throughout the game, they only change in intensity and the way his actions manifest from them.