Meruem initially is seen as a cruel and violent leader. He considers himself superior to all other forms of life and feels no concern for his dying mother after his birth. Meruem is also merciless, killing and cannibalizing any Chimera Ant he deems as disrespectful.
He easily withstands thousands of attacks launched at him by Netero, one of the most powerful human fighters in the world Meruem's speed was such that it allowed him to dodge Netero's supersonic blows to close in on and rip off Netero's limbs before the Chairman could put up his guard.
Its not because she's powerful, or the most evil of evilest people. She's not even a bully. Nah, Nakajima is one of those people who can't stand other people being capable of success other than her. And her kindness to Ogiue is hollow, even creepy. She is extremely jealous and loves to stir things around if only to bring despair on her target.
While his appearance isn't exactly intimidating, his power is absolute. In the HxH universe, power can't simply be measured by the amount of Aura (energy) you have. However, Meruem's Aura was so powerful, that several strong characters were left in despair by just sensing it.
His power was so terrifying, that even a powerful and a confident individual like Welfin was tremendously terrified by his presence alone. In fact, just by standing in front of Meruem, he was so terrified to the point where he aged several years and withered completely.