He didn't live long enough to know the outcome of WWI and he was very sick during his captivity, however, in 1916 a military psychologist named Pappenheim interviewed Princip several times over a period of four months, and recorded the conversations.

This is what he had to say:

In solitary confinement very bad, without books, nothing to read, no communication. Used to constant reading. Misses reading most down here. Sleeps at night four hours at most. Dreams a lot. Beautiful dreams. Of life, of love, nothing horrible. Thinks of all, especially of circumstances in fatherland.Heard something of war. Heard a tragic thing, that Serbia is no more. His life is hard anyway, Serbia is no more, my people will have a rough time. World War was inevitable, didn’t depend on that. He was an ideal man, wanted to avenge his people. The motives, revenge and love. All his youth in such a revolutionary mood. Talks of anarchist pamphlets that led him to assassination.Thinks differently today, thinks social revolution is possible in entire Europe, things change. Doesn’t want to talk about it in front of the warden.

(When this interview was conducted, Serbia had recently — four months earlier — been conquered by the German army after successfully holding out against Austria-Hungary for over a year.)

In a later interview Princip said again that he felt that a war between the Great Powers would have been inevitable sooner or later.

He cannot believe the World War was a consequence of the assassination; cannot feel himself responsible for the catastrophe.

He was a naïve, idealistic teenager who believed that striking a blow against the hated Austrian oppressor would unite the Serb, Croat and Slovene people into one nation. He doesn't seem to have thought beyond that; he hadn't expected to survive the assassination or to live the consequences of his actions so, it makes sense that he never put too much thought about it.