I've been trying to see where in the text the game actually says that FATE killed(A)/tried to kill(H) Serge for the purpose of trying to release the lock on the Flame and disable the Prometheus circuit. Based on what I could find, which is the same quote from Ghost Crono that a lot of people are quoting, I think an alternative interpretation is possible.
Ten years ago, it was Lynx
who tried to kill you at
this beach.
After Prometheus broke the link
between FATE and the Flame,
FATE tried to eliminate any
obstacle that stood in its way!
...
The elimination of the
Prometheus circuit's lock
on the Frozen Flame was
everyone's top priority!
...
Lynx and Harle abducted
Lucca, who alone could
release the Prometheus lock
that guarded the Flame...
But the whole attempt only
ended in failure.
Then, they just waited for
you to appear instead!
So it seems possible that FATE/Lynx never thought that killing Serge would release the lock. For that, they were relying on the first plan, kidnapping Lucca and making her do it. The killing of/attempt to kill Serge was merely to "eliminate any obstacle that stood in its way", meaning they could see that Serge would become powerful enough to challenge them later.
In this interpretation, when the Lucca plan failed they must have breathed a sigh of relief that they *hadn't* succeeded in killing Serge (at least not in both worlds, which FATE now knew was enough since it knew Serge would eventually cross dimensions), since the Serge body-swap plan was now FATE/Lynx's only chance to release the lock on the Flame.
This is, of course, unless there is more text in the game that I missed that makes it clear that the drowning was indeed intended to directly release the lock on the Flame. Does anyone know? Search terms only took me so far.
