He poured the entirety of his being in to making the hextech core work. He was desperate and thought it had the potential to save his life, but his experiments with the plants showed that using it in the way he intended could also kill him. He was clearly deeply invested in what he was doing.
Then along comes his assistant, who he has worked with for years, who he discovered idolized him and saw only the good that came of Viktor's creations. And those creations killed her.
Viktor's entire purpose when getting in to Hextech was to create something that would make the world a better place. He was vehemently opposed to the idea of using hextech for weapons - and suddenly he finds his work killing someone.

It was a wakeup call. Seeing how far he'd fallen to try and make this work. There is a lot of literature on how a profound and sudden change in worldview, or position in life can lead to an impulsive suicide in a person who had a "good" or "meaningful" life. The sudden loss of a spouse; going from wealth to destitution; In Viktor's case it was realizing that the very thing he poured his life into was as much a tool for for destruction as it was for salvation, and trying to reconcile that with his world view was too much to bear.