Regardless of how bloodthirsty and psychopathic he is in battle or when being forced to retreat from battle (he was pretty pissed off when Abaddon had him teleported away from a battle but left other berzerkers there to die for example,) he's still capable of spending long periods of time not killing many people at all. He can still form alliances, maintain his wargear, command warbands and captain ships. He's only doing that to further the goal of killing more people in the future, but he's still lucid enough to make such decisions.

Most World Eater's are the same, they spend ages in a state of restrained fury occasionally butchering slaves or fighting duels to keep themselves under control until they get to warzones where they can let loose and abandon most of their rationality and reason. They aren't foaming at the mouth 24/7 and killing anything that bumps into them. Most of the time they're just really angry marines.