Hojo "created" Sephiroth in a sense, but that doesn't make him responsible for his son's actions. Once Sephiroth discovered the truth of his origins, he made the choice to kill. Jenova didn't "do" anything to him, no blame can be placed on her really.
True, but if it weren't for Jenova's cells and Hojo's experimentation, Sephiroth would be a normal human being. It's hard to tell whether Jenova has control over her cells or not, but I have to think that Hojo was at least somewhat aware of the possible consequences of his experiment.
The experiments were done in order to create a super-human being, that much is clear. Sephiroth was, for all intensive purposes, a success. Before he even came in contact with the Lifestream, he was already the most powerful member of SOLDIER (see the Nibelheim flashbacks).

Think of Cloud - when he arrived at Mideel the doctor said he's had an immense amount of Mako-drenched knowledge put in all at once. Cloud pretty much caved in and contracted Mako poisoning. But Sephiroth's stronger than that. I think, given five years, he could quite certainly make use of all that "knowledge" he comes across. And we know he did, because he suddenly knows all about the Black Materia and Meteor (and the fact that he flat out tells us he gained the knowledge). There's never any indication that Jenova's involved somehow.
Both Cloud and Sephiroth had similar experiments performed on them. As members of SOLDIER, both had been given Mako treatments and Jenova cells. There is one crucial difference however; Sephiroth was injected with Jenova cells before he was even born, while Cloud recieved them after the Nibelheim incident. I don't know if there's much evidence in the game to support it, but it's my opinion that this more intense exposure to Jenova cells may be the reason that Sephiroth can handle all that Mako while Cloud cannot.

~MJE~