Gavrilo Princip, the man who actually killed Ferdinand, died while WWI was still ongoing. But many lived for decades afterward. Vaso Čubrilović lived until 1990. Cvjetko Popović lived until 1984. Did any of them ever say "Maybe we went too far" or something like that?
Not to discourage any incoming answers, but a similar question has been nicely answered by u/JDolan283.