So, whether or not animus magic causes you to lose your soul has never actually been explicitly canonically confirmed, which I really like, since it leaves room to decide what you think as the reader, and everyone gets to have their own opinion. I've thought a lot about this, and this is my personal theory, and the mindset I'll be approaching Shadowhunter and Darkstalker's magic with throughout this book. :)

So, in my opinion, animus magic on its own isn't bad--Darkstalker is gonna go on using it for the rest of his life, following the basic ground rule of "I'm not gonna use my magic to help in any kind of killing or violence or to alter anyone's free will" and never turns evil, same as Shadowhunter (after a lot of character development lol). 

It's how you use it that corrupts you--good spells make you more likely to keep doing good things, and bad spells make you more likely to keep doing bad things. 

I also like to think there are physical consequences to doing too much magic, especially with negative intent, which is what I tried to show with Shadowhunter. For her, animus magic is sort of like addiction or self-harm--after doing a spell, she gets this super intense rush, and then because she's using so much power so frequently, after that initial adrenaline rush she just goes kinda numb, because it's taken so much out of her. A lot of her negative character arc is more of her own mindset towards her magic, and using it irresponsibly.  (She tells herself her soul is good as gone because she's been told her whole life that's what'll happen to her someday--and proceeds to do more and more bad things because she may as well, and to a degree, it's an easy way to not have to take any responsibility for her actions.)

Darkstalker wouldn't experience those kind of extreme physical consequences for doing magic like his daughter does, because it's not actually in him, it's in a scroll. (Although I do imagine drawing on that is mentally draining and he would get pretty tired just doing spells all day, in the same way staring at a computer or studying for a test is draining--it just requires a lot of focus and concentration.)

Later on in this book, Shadowhunter gets her magic back with some limitations to make sure she doesn't go and murder people again, and starts to use it for good things, like making Permafrost a necklace to always keep her at the right temperature, since she's always kinda hot and stuffy in the Night Kingdom--or making it snow for her whenever she misses home. And those spells feel completely different, they give her like this super warm, giddy rush, because they're done from a place of love.

Darkstalker doing things like repairing the city, and helping keep the kingdom safe don't cause him to turn bad because he chooses to use his power that way out of a place of love--if anything they make him into a better person, or more likely to do good things in the future. 

I also like to think you can sort of sense good spells and bad spells--like how in canon characters note when interacting with animus touched objects, some of them feel bad and wrong, and others feel safe and good.

Obviously, this is a super black and white, simplistic way of thinking about things, but this is my general framework.

TLDR: like any other power, it can be used for bad or good. Good spells make you into a better person over time--and bad spells make you spiral into being a worse person over time.

You don't have to agree with me on this, this is just my personal take. Please do not yell at me in the comments, I'm sorry if this is confusing or poorly worded, I've had a very long day and I'm tired. :)