I ❤️ Art

Sometimes I wonder why Ken is so desperate to keep Mel alive and uninjured when if she does die he wouldn’t have to worry about keeping such a big secret anymore

We don't really know the full circumstances as to why Mel's in his care, but we do know that there was some kind of deal between him and Temperance, most likely to keep the other virtues off of Ken's back.

Could it be that he's over the eternal life thing? Or that he promised to raise her in exchange for his freedom? Or if he initially planned to kill her but just couldn't bring himself to do it?

We don't know. And we're probably not going to know until closer to the end of the show.

But what I can tell you is that he loves her. Sure, he calls her a brat, argues with her, and does some things that constitutes as emotional abuse, but he still loves Mel. If he didn't care about her and would be "better off" with her dead, he wouldn't have gone through all of the trouble to protect her as you've said.

Ken views Mel through the eyes of a parent. He doesn't just see her as the prophesized human child, he sees her as his daughter. The same way he views Breadhead as his son. And it's his role, as a father, to protect his children. Ken has more leeway with Breadhead, because Breadhead has the mark of the Black Hand, and can't permanently die. He has to be more cautious with Mel, because of her mortality.