And guess what? When you get into that mindset, it becomes easier and easier to accept greater and greater sacrifices on the altar of your ambition. "I will slay 100 to save a thousand. I will slay a thousand to save a million, I will slay a million to save a billion, I will slay every living and sapient being in all of existence to build a new existence that isn't as terrible."
Kiritsugu's ambition was never portrayed as being right and proper, nor was it considered right and proper when EMIYA took it up. It was explicitly monstrous the steps they were willing to go, how many atrocities they were willing to conduct to achieve their goals, using the final endgoal of "Saving People" to justify it.
And guess what? In every event where someone followed that ideal to the end, it came to the same outcome. The death and ruination of the one involved, and the suffering of their loved ones.
Even Odyssial was not exempt.