First, some random comment:
I am pretty sure Natalie is the murderer, or at least helping one. She didn't call the police even though she said she would, and she seems desperate to to keep you at her side, likely to give herself an alibi.
Witch, I want you repeat the following statements in red, if you can:
Simon is dead.
When we checked Simon's pulse, he was already dead.
When Alfred unlocked the door to the office, Simon was already dead.
Simon was murdered by another person.
Next, some blue truths:
After the murderer had killed Simon, he never left the office. The murderer hid and stayed there. The other murders were either done before Simon's, or commited by someone else.
The murderer entered the Simon's office by unlocking it with a key, killed Simon, then locked the door from the outside. The actual key never entered the room though.