By killing those five Opfer after entering the facility, Fen's timer with his Soul energy enhanced strength extended after his body absorbed the fragments of their memories and added them to the mental space alongside the rest of the Void Spawn's memories. He wouldn't get the opportunity to experience their memories if he used them to fuel his strength, (which might be a hidden benefit, if anything... Fen never got used to how unnerving it was to see the world from another person's perspective) but at the same time, he was still forced to feel their emotions when their Soul energy was used to strengthen his body.
There was a popular theory back on Earth regarding cell memory, and how after getting an organ transplant, some patients experienced certain personality changes that appear to have originated from the original owner of the organ- that theory was never proven true, but in the case of the soul and energy originating from absorbed memories, it was very much accurate.
