I mean I'm not a philosopher or an expert but I don't think you can read Capital and not conclude that Marx is at least partially motivated by moral outrage. Page after page of examples of children doing dangerous and exhausting work, rigorously documenting just how little food workers are getting, he even waxes poetic at times to make it clear how bad he thinks the capitalists are when they defend these realities as necessary or moral.

It's not strictly necessary for the theory to be useful (hell, it seems like the worst capitalists clearly understand the theory, even if they don't articulate it in the same terms) but Marx isn't, in my opinion, neutral as to which side is good. Even if he would say it's best to assume a scientific detachment I think he has picked a side, and not merely for material reasons.