Darkness isn't evil, that's just what mankind and humans refer to it as. he refers to it as awe-inspiring and glorious, and he has accepted it, and so it courses through his veins, at the cost of his sanity, the more level of darkness he is in gives him more power, but, just like I posted, too much power will make him go fully insane, and if he leaves that power too quickly, he'll pass out

EDIT: It really only is because of Logan's realm of darkness, because thinking about the trade-off of sanity and power, since the realm is kinda made purely of darkness, he'd get powerful and insane