In season 3, Malcolm caused the feud between Oliver and Ra's, and he also played a part in stopping them by providing Oliver with vital information on how the league works and their initiation process. He also saved Team Arrow's lives by providing them with a cure for the poison that Ra's put in their cell. In season 4 he revealed Williams existence to Darhk, which resulted in him having to leave Oliver's life, and he also played a big part in getting Darhk out of prison and getting his powers back, which resulted in Laurel's death, and of course Genesis still going ahead.
It would be nice if he had development or an arc though.