Mina's a good girl. Despite how bad her former owner treated her and Luna before, she still believes a loss of life is a sad thing, so being even the slightest bit happy over someone's death would not be something a 'good girl' should be feeling, at least that's what I assume her thoughts were here.
At the end of the day, though, we're all imperfect human beings. We all have flaws in places where what gives us joy, that much is normal. Let's just hope that after having that much introspection of herself, Mina can learn how to process and acknowledge such feelings before figuring out how to deal with them down the line, and not let such things define her as an individual.