The sheer mass of the legionaries attacking Perseus's vastly outnumbered phalangists at the center quickly began to make them buckle.
The legionaries were far more aggressive than their counterparts, and not only were they able to absorb more casualties, but they were also able to exploit far more gaps and opportunities than the opposing phalangists, leading to a hugely disproportionate amount of casualties.
Even Perseus standing way back of his army could clearly see the blue armored legionaries starting to reave through his men like they were cutting wheat, showing very enthusiastic smiles as they did so.
There was not even a twinge of regret in them as they hacked through, instead actually appearing quite elated, for in a battlefield, the best enemy was a weak enemy.
They were easy to kill, and more importantly, facing them it was easier to stay alive.