Biologically speaking, these corrupted creatures were unable to feel any emotions like fear since that would only slow them down but that didn't mean their self-preservation instincts were non-existent.
They would be greatly suppressed in battle but outside of that they would do whatever was necessary to make sure the colony continued to progress. And this was a prime example of that.
They were sacrificing hundreds of incomplete grunts and pouring those 'resources' to forcefully evolve another creature into being, one that would be far stronger than anything else.
If this was a game then that creature in the bog would be classified as a hero unit.
Ordinary grunts were unable to use spiritual energy or mana and depended solely on their physical bodies along with the bioengineered weapons attached to them in battle. But special creatures like this were the exception.
There would be no way to tell what it would be capable of in the end.