Can I tell my friend her actions were shitty in this conversation
Why do you feel that her actions were subpar? Tell me more.
Her ways of treating people were scary and I found them to be completely wrong
It is good that you are voicing how you feel here. Holding all that in will simply make you go crazy eventually. You are doing the right thing in being honest.
I don't feel I'm doing the right thing. I'm mad that I'm going to lose my best friend.
I don't think you are going to lose your friend. If you tell her calmly as constructive criticism, it will not come off as harsh. Just make sure your tone is gentle too, so that she doesn't take offense.