Found: Thriller, killer, giant tree poisonned, end in a hotel (?)

Hi, i read this book in the last 8 years, but i'm not sure it's a story in the 21century, i can't remember if there were cell phone, computer...
The story is about a man, killer. At the beginning we see him enter in a park, and go straight to a big tree. He get a syringe et plant a needle in the tree. We don't understand what he's doing.
At the end of the book, he stands near a window (i think it's in a hotel, after he triggered the alarm, to be alone upstairs, but not sure at all...) and he sees the park with all the leafy treas, but there is one tree without leaf, the one he killed at the beginning, that let him see an appartment, or house. We understand he created his line of sight and he takes his rifle.
I can't remember what was the police doing ; i didn't have the feeling you have normally when you know the man is a killer, and you want he to be arrested, maybe he was a spy.
I really hope you can help me :)
Thank you !

I'm positive that this is a Jo Nesbø thriller, and I want to say it's The redbreast. If not, it's definitely one from the Harry Hole series -- it's been a while since I read Nesbø.

>2 Petroglyph: I'm so happy :) You're right ! What a good memory you have ! Thank you so much, i was becoming mad...!

>3 Agacciu:

My pleasure!