Not becomes one, but joins the Demon King after a change of views or is just plain evil. Anything is fine.
Are you asking if the MC switches sides throughout the novel or is of the evil path Obligatory Warlock Of The Magnus World ?
Nidome no Yuusha Werewolf Reincarnation Demon King & Hero Hataraku Maou-sama Maoyuu Maou Yuusha Sairin Yuusha no Fukushuu Hanashi Maou no Hajimekata Katahane no Riku
What Came to Mind During My Third Time in Another World Was to for Now, Get Naked. MC bum rushes the demon lord to get things over with only to end up joining hands.