Cutting isn't really bad. The cutting itself is not the issue. The problem is that you're cutting instead of finding healthier ways to manage the deeper issues that you're going through.

Most of the therapists I've spoken to about cutting see it as something to wane off of and replace with habits that help you work with and face the underlying issue, rather than drowning them out with pain. These same therapists normally see all forms of escapism and distraction as almost as bad as cutting, because at the end of the day they aren't helping you move through or address your issues at all.

Not only that, but with some people, self-harm actually makes the underlying problems worse in the long-run despite providing an initial release because it makes those individuals more comfortable with hurting themselves. This is sometimes carried over psychologically, where these people become quicker to berate themselves or view themselves negatively because they have less inhibitions against attacking themselves. This isn't always the case, but it can be.

I'm also not a professional, so don't take my possibly distorted words from possibly underqualified or incorrect clinicians for granted.

(Edit: I should mention that I've never SI'd, but I've been fairly close with people who do. As such, I can't comment on how this lines up in my experience. From what I've seen, the people in my life do start getting better when they stop self-harming, though.)