K. made the rational observation that no one would pay attention to him unless he did something drastic to force it. He'd just be a forgotten loner who scribbled in notebooks.

A politician might kill or harm many more people in the course of pursuing their objectives but they'll always have the story on their side provided they win.

K. had to understand that the achievement of his objective, publishing the manifesto, would likely lead to his capture, but he did it anyway. He sacrificed himself in the end.

If you believed you had information that would save the planet but you had to kill some innocent people to make it known? Utilitarians would say yes because saving billions of innocent lives is more important.

K. probably saw himself as someone taking revenge on an evil system that had harmed him. He's one of the more fascinating characters of modern history in that he was so effective in attaining his goals with violence.