Del Toro's first task in adapting the saga was to get a screenplay that could convey all the important elements of "Hellboy" in around 90 minutes. He needed a story that would appeal to fans of the comic book and connect with audience members who had never heard of "Hellboy."
Screenwriters normally handle the job of screen adaptation. They often have no familiarity with the subject outside of the research they do to write the script. This lack of familiarity often comes across in the final product. Guillermo decided the best person suited for the job of adapting the script would be a true fan. The biggest Hellboy fan Guillermo del Toro knows is Guillermo del Toro.
Writing the screenplay himself gave del Toro total control over which story elements from the comic were fleshed out and which were let go. It also allowed him to carve a plot that was true to "Hellboy" while still fitting it into the format of cinema storytelling.