So, I generally operate from the perspective that all Parts -- even ones that are suicidal or appear to be "evil" -- are there to protect the system. Suicidal parts, for instance, are trying to protect in the most ultimate way they know how: by literally ending everything... that is the ultimate protection.

Parts that appear "evil" are, I believe, still there trying to protect the system in some way. For instance, perhaps by embracing violence & disconnecting from empathy, they believe they are stronger and, thus, safer from other violent people.

Even parts that reenact trauma or cause physical harm tend to be doing it for some sort of protective reason -- either in an attempt to "undo" the trauma by reenacting it and bringing it to a different conclusion or by trying to take control of the trauma by owning it and making it their choice. Stuff like that.

I do not believe in demonic or evil Parts anymore -- I think your feelings towards this Part should be examined. Your view of it as evil is important -- it shows an area where you both could grow in communication with one another. Keeping in mind that this Part may actually WANT you to view it as evil, because being viewed as "evil" could make it feel strong, which makes it feel safer, which is, again, a form of protection.