When Phantom wanted to change his Initial Gene, he must first get rid of the old one. That had to be done beforehand so that when he fought the monster of his choice, the Evolution Tree would instantly make contact with the monster's genes.
If Phantom deleted his Initial Gene only after killing the monster, the chances of successfully replacing the Initial Gene would be ten times lower, because during the battle, Evolution Tree would not try to find a replacement for the void within itself, treating the monster as a source of simple genes rather than a new core of its power.
Katrin knew this, as well as all other details related to Initial Gene replacement, it was the original goal of this mission, it was her dream, so she prepared perfectly.
Adam also knew what needed to be done on his part. Katrin wasn't strong enough to kill Essence Scarlet Pyromancer alone, her aura was strong enough, but she was the support.
