"If JMS had not mentioned the hole in Sinclair's mind, what would
have been the reason for the assassin to try and kill Sinclair?"

Hello...did you see the same movie that I wrote? The assassin was
not there to kill Sinclair. He was there to kill Kosh. He tried to kill
Kosh. He tried to stay AWAY from Sinclair, did everything in his power
to avoid Sinclair, ran from
Sinclair, and only finally encountered Sinclair when Sinclair came after
HIM. Then it was nominal self-defense.

Had the "hole in the mind" reference never been made, it would have
been clear -- at least clear to every other carbon-based lifeform who saw
the movie -- that the assassin 1) came to try and kill Kosh, 2) in the
hope of disrupting the purpose of Babylon 5, with the added benefit of
3) if he failed in his mission, setting up Sinclair to take the rap for
his actions. At the very end, rather than be captured and interrogated,
the Minbari assassin killed himself with an implanted bomb. His comment
to Sinclair at that moment was more of an "Up yours" comment, designed to
shatter Sinclair with the knowledge that he knew something Sinclair
didn't.

You keep saying he was there to kill Sinclair. He wasn't. He didn't.
He didn't try. It makes it hard to have this conversation with you if
your comments don't touch reality at any two contiguous points.

jms