Griffith from Berserk wasn't manipulated by the God Hand. If he was, the entire story, all the hate and all that Guts has fought for, would be meaningless. It wouldn't be a personal conflict anymore, but yet another story about a hero being besieged by faceless villains. That Griffith made that choice, and personally destroyed Guts and Casca, is what makes the story meaningful.
And the evidence is strong that becoming Femto wasn't the result of manipulating Griffith, but of showing exactly who he was all along.
Regarding Griffith's personal character, well, it's clear he was a bad seed even beforehand. While it might be argued that his attacks on the royalty were simple retribution, Griffith is very, very good at these things.
Someone who leaves others to burn to death or arranges for the kidnapping of a child to be used as a bargaining tool, regardless of circumstances, can't be seen as a fine, upstanding individual. There was something dark in Griffith, or destiny would not have chosen him to become a member of the God Hand.
But let's look at the Eclipse itself. At that time, Griffith was a ruin of a man, mutilated in body, mind, and spirit. About to commit suicide. The Hand offered him a way out.
Some claim Griffith was not in his right mind when he made the bargain, and therefore the results can't be held against him. But, while Griffith was worn down to almost nothing, the Hand appealed to what we had seen of Griffith before, and they did not manipulate Griffith: they reminded him.
And they reminded Griffith of things that were completely true. First, that he had ambitions, dreams that now would be impossible to fulfill for a crippled man in a medieval society. All he could do is sit in a chair and be fed by Casca, a scenario which, no matter how bucolic it was, represented Griffith's worst nightmare.
They told Griffith he would only achieve his dreams through strife and bloodshed, and they still were not lying. The Sacrifice would be just another example of the way that kingdoms were founded on mountains of corpses, which is a fact of Midland.
Griffith was never lied to at any point, but simply shown the actual choice before him: make the Sacrifice, or lose his dream. He chose the dream. He said "yes" to the wholesale slaughter of his men, but it was consistent with the Griffith we saw, what Griffith would have wanted.
On some level Griffith must have valued his men, or the Sacrifice would have been meaningless, and yet Griffith still made that choice. We have seen before that he has the capacity for evil, and that ambition is what drives him above all else. Even his own body is a tool for achieving those ends.
Further proof is that as Femto, Griffith singled out Guts and Casca for special tortures. Femto would not have done this if he did not possess Griffith's hates and wants—if he wasn't Griffith. The two beings are the same, and what Femto does is only an expression of the evil that Griffith already had inside him. Griffith was only told the truth, and reacted as one expected him to.