User blog comment:TheDoctor230/2024 Barry/@comment-27700324-20170503133659/@comment-24152851-20170503134249

My guess is it's a bootstrap paradox. Iris' death caused Barry to become despaired and become Savitar, but he could only do so because Savitar killed Iris to begin with. There might not be a true reason as to why Savitar did it other than "history says so".