Well, he knew for a fact that Jenova wasn't his mother, that is made rather clear at different points in the game, notably when he tells you he knows that.

The only real motivation I can see for Sephiroth is wanting revenge on the world that wronged him, which in all reality was limited to Shinra. Wanting to take out everyone else in the process, people who weren't remotely involved, sounds like a tantrum to me.

Basically the prospect of power just happened to fall into his lap and he decided to take it?

And it sounds odd that he wouls be partly comprised of her nature. If that were the case, then wouldn't the other Soldiers, for having Jenova Cells implanted in them, have similar urges? If not, why only Sephiroth? Is it because of the differences in how he was created?

If so, why did it take so long for him to feel these urges? From what I could tell, it wasn't until the Library scene under Shinra Mansion that these feelings ever started to begin with.

I'm not asking to prove a point, I'm asking to get a very clear reasoning behind Sephiroths motives so I can clearer argue the point.

To me, all of these questions are important to be answered to do such a thing.