7
The police were unable to find Frankie's identity, so they could only start their investigation through his employers, the Xenakis brothers. If they had a grudge against Sebastian, it was possible that Frankie murdering Sebastian was to help them take revenge.
However, this clue in the investigation soon ended because the Xenakis brothers and Sebastian had no grudge against each other at all.
Jonathan was in the last batch of Grade 12 students who were taught by Sebastian. He was the monitor of Sebastian's class back then. He had good academic grades and strong performance. Later, he even studied until he achieved a doctoral degree. Thus, it could be said that he was Sebastian's favorite student.
It was even more unlikely for James. He was not even a student of Sebastian, and the only connection between him and Sebastian was Jonathan.
Naturally, Frankie did not kill Sebastian for the sake of the Xenakis brothers.