There's a huge assumption here, that all nomadic horse cultures around the world and throughout time are "incredibly brutal" to any neighboring settled agriculturalists. You have a stereotype stuck in your head, and it doesn't correspond to history, so this question becomes unanswerable.

For example, I will point out that the Sarmatians, as described by ancient authors & modern archaeology, traded with the cities around the Black Sea, and the Scythians hired as mercenaries to be the police of ancient Athens.

So you need to tell us which nomadic horse cultures you specifically are asking about.

There are two theories I can think of tangentially related to that question of yours (which as Zither pointed out is footing on a questionable premise). The Persian prophet Zarathustra preached that it's because the nomads were created by Ahriman, the lord of lies, rather than by Ahura Mazda, the lord of all that is good and proper. The lover of Ahriman has also been pretty nasty. She created menstruation. Source: Igor Rak - Thus spake no Zarathustra ("Чего не говорил Заратустра")

Plekhanov, a Russian Marxist, explains not nomads but hunter-gatherers: Since they have zero use for slaves people they caught they'd simply cook and eat (Tupi indians) or adopt (Pawnee indians). You could extend that idea to nomads. source: G.V. Plekhanov - Monist View on History.