one of the more important parts is that the notepad doesn't have any blood on it, meaning it was placed there after the murder and probably isn't the victim's at all. The notepad either belongs to the son or Keiko, because Yuki wouldn't have one as she uses a computer. As it's not the professor's, but still contains a "dying message", it's probably due to whoever left it covering up for the real murderer. Because the order of their leaving was Keiko, Yuki, Akito, Keiko couldn't have found the body after the original murderer to leave her notepad, this leaves the one who covered up the murder to be Kentaro Akito himself, which is also why he fled the scene afterward.
As you hinted at love being part of the riddle, and there's a cover-up involved, it's probable that Akito was covering up for someone he loved. Which, leaving out brotherly love of long-lost-brothers or homosexual love, negates the night watchman from being the true murderer.
This leaves the two women to be the possible murder suspects, and the conditions of the victim's death. There's the issue of the pens (which, I'm guessing is an English-related dying message but I can't find out what it is), and the fact that he was stabbed from behind, even though he was sitting at his desk in his office, where people would normally be in front of him (not to mention, if he's sitting in a chair, a back-attack should be impossible aside from the neck), so he probably has some relationship with the killer as well. Love triangle omg?!
it is also possible that the notepad is in fact the president's notepad as the first page was ripped off (is there traces of blood on the sides of the notepad?), meaning it was supposed to say "Keiko" in the first place.
so I'm going with the murderer being Keiko. How close am I? :V