I don't think she's actually evil in a sense that she's using him for nefarious ends. It seems like she actually likes him because when he asks her about why, she gives him a very sincere look when she tells him that he is very kind.
We see that she becomes potentially upset at the thought of him leaving the town. Most likely she is going to start killing the townsfolk who bother him in order to encourage him to stay.
You are correct - she's not entirely an evil monster. Ayame clearly has positive feelings for and might be sincere about Kao, but I would say that's her Yandere traits at work. Like her stalking him. Still, Ayame definitely is evil in the sense that she has no issue murdering people as a means to her goals - not to mention the way she killed Kao's old man, she's quite brutal. The question is why or how she came to like the MC. As I mentioned, the two probably share a past that Kao isn't aware of right now.
1. She actually left the talisman at the scene of the crime
Ho? You are right, I didn't notice that... good eye for details. Now this is interesting.
Because it leads to the question why she even did that. On one hand it's Kao's charm, so leaving it at a crime scene might end up as circumstancial evidence for the police. Forensics could be lead to the former owner of that charm if they can find Kao's DNA on it. Well, if that's even possible given that other people touched that thing all over and it's probably spoiled. On the other hand, leaving that charm after it had been lost might cause something with Kao himself - if he realizes or at least believes that it's his charm. Because in a sense it actually worked - his old man was clearly a violent and bad guy.