Well it relates to the themes of the Chimera Ant arc in general which were that of gaining or losing humanity. The Chimera Ant's are for the enemy, the threat but several of them grow as characters in an incredibly short amount of time, some mature and some don't, others realize what they're doing conflicts with their more human feelings and thoughts, some revel in it and one even learns to fall in love and care about someone more than himself.
Whereas the human's in this arc often go to the limits of their abilities and do what are horrible things. Gon absolutely loses himself in his despair and anguish over not only failing Kite but then being unable to heal him back to normal. He quite literally turns into a monster in a suicide pact to sacrifice everything he has to just kill Pitou, he killed himself by Nen. Netero is much the same. Meruem wanted several times to try and talk it out and was even willing to learn as he became more connected to Komugi and thus his own human side.
Netero wasn't having any of it and as well was also a part of the Dragon Dive that destroyed much of the palace and surrounding area and killed who know's how many random people. Netero even has a moment where he ponders if he should possibly take the higher road and try and reason with the CA but he quickly quashes that line of thought. Its both its too late and I think Netero realizes that like the Native American tribes and the Europeans, only one side was going to last no matter what. So he made the decision to end the King no matter what, the fight was just his pride and want to test his own abilities at his old age, the Rose was to actually seal the deal.
