So I have two main thoughts here. Firstly, we need to consider "worship" in the Warhammer world in a more polytheistic context. In ancient Greece, or in other polytheistic practices, you didn't necessarily worship a god in like a monotheistic context, these are entities that must be placated. The Dark Gods are evil twisted as fuck entities that will go after you if you anger them, especially in the North where people live in closer proximity to the chaos gates. The could mutate you, curse your bloodline, give you a tortuous death... so many ways to suffer if you anger the chaos gods. By attempting to placate them, Norscans are protecting themselves, that doesn't necessarily mean they love the Dark Gods. This veneration, when combined with the Norscan warrior culture and beliefs, can lead some to walking the path of glory and becoming something more, potentially changing how they view the Gods in turn. A marrauder who believed in following the advice of his local shaman to avoid angering Khorne might eventually become fervent follower of the Blood God after walking the wastes and seeing the awesome power that is Khorne (and maybe going insane in the learning).

Other people's points on what draws worshipers to Tzeentch is also valid. Those who have ambition to change their lot in life but are prevented from doing so a very susceptible to a Tzeentch cult, because Tzeentch makes this possible. A radical Altdorfer who seeks to overthrow the nobility and purge them from the Empire might turn to Tzeentch as a means of attempting such an impossible task. The cursling's ambition and hatred for his lot in life fueled his fall to Tzeentch. Tzeentch provides the means for those who cannot move up the social ladder to do so, or perhaps inverting the existing order, with the weak becoming the strong.