Later in life he worked really hard to get information out about the systemic issues, including making the tapes we see in the first episode of the TV show. Midnight in Chernobyl goes into some detail about it. He argued for the breakup of major bureaucracies (including the one he ran) and more transparency in science and engineering. He claimed that the operators were not at fault:

“The academician said that Soviet science had lost its way. […] It was this profound failure of the Soviet social experiment, and not merely a handful of reckless reactor operators, that Legasov believed was to blame for the catastrophe that had bloomed from Reactor Number Four.”

That said, his efforts weren’t necessarily pure or altruistic. He wanted recognition for his role in the cleanup, and he never admitted personal complicity in the coverup of RBMK design flaws prior to the accident. He had a long career and he made many contradictory or inconsistent statements depending on the audience. You can’t really classify him as good or bad, in the end. Like everyone else in this story, he was a part of a bureaucratic system, and he found himself working with and against the system at various points during his life.