Medieval warfare wasn't really ideological, it was rather integral to their political and economic systems. Propaganda wasn't really necessary either, the people having no real say anyway. Ultimately, people made war because, in the case of the rich, they wanted more land and wealth, in the case of the ambitious, because they wanted opportunities for loot, pillage and adventure, and for the poor because they were forced into it. In many ways, not all that different from today really, but they were more stark and honest about it. If there was an "ideological" tinge it might be over something like religious differences but they wouldn't map onto modern ideas of morality very well and that was usually just a flimsy justification anyway. It's better if you don't think about present-day rhetoric like armed combat being over freedom or bringing people democracy and just think of it like ant or bee colonies fighting over space. There really doesn't need to be any philosophy going on there.
What you're concerned about, how well a serf would have been treated, was up to individual lords, or just the passing mood of a lord, and not really by kingdoms. So, the only thing you can do is be a good lord yourself, regardless of faction, and try to become king and perhaps only promote honorable lords and then eventually take over the map. Of course, the latter part means even more war and fighting than there'd normally be.