>>3122012As a really poor peasant kid he would dream about what life in the castle above the shitty backstreets he lived in was like, and decided one day he wanted his own kingdom. Not for some altruistic purpose, ideal of justice, or other noble goal (though arguably, he would of made an excellent and just ruler and maybe still will). He simply wanted one, and focused his whole life on that dream and was willing to use every life under his command solely for that sake.
He didn't become an uncaring monster, he always kind of was one. He just wasn't needlessly cruel or abrasive to his men or innocents or whatever. Griffith just valued his dream more than anything or anyone (bar Guts until the Eclipse), and most of his men knew that and signed up anyways (being eaten ripped apart by monsters and condemned to Hell probably wasn't in the contract though). He does show a few moments of weakness and almost-but-not-quite regret though (the kid, Casca finding out about Gannon, build up to the Eclipse).
I strongly encourage you to read the manga, it's very good.
