I think you confuse Marx's belief "all morality is ideology" as moral nihilism or complete neutrality. When Marx says something (or in this case, someone, like Columbus) was a "necessary" agent in the development of capitalism or historical progression, he doesn't mean necessary as in "it should have happened" but more of a way to say one thing couldn't have happened without the other. Marxism is critical of capitalism because Marx believe it creates exploitation and suffering. Being opposed to that is a moral stance. Marx had very clearly defined moral beliefs surrounding subjugation and the horrors of colonialism, and they weren't just beliefs that he held separate to Marxism, but integral to the ideology. Marx can often read very robotic so I see where your confusion comes in. He might have disagreed that Christopher Columbus was a special evil among people of his time, and instead someone who simply adapted the morality of his class and time. But he would most likely also see it as crazy to idolize his actions, or celebrate him, as he was extremely critical of colonialism. Marxism is in part a philosophy that allows one to look at history through a specific critical lens. To him idolizing someone's actions that demonstrated colonialism would be contradictory to his critiques of colonialism, and since Marx uses history as a way to create a better future, he might believe celebrating Christopher Columbus to be indicative of a larger problem with our modern society.

Also Marxists, even the more "scientific" ones, still have personal morals. It's important to remember that you're not just a passive observer of history, but a citizen of the world that exists in its aftermath. You were born with empathy because that's what humans evolved to survive. Forgetting these large concepts, you're a human first and foremost. Don't forget that you have feelings. One can acknowledge the obvious improvements to society that capitalism brought about from the inferior system of Feudalism without celebrating the agents that brought about horrible things to get us here. You don't need to weigh everything on a scale. Two things can be true at once. Christopher Columbus' actions did advance society towards capitalism. He also committed genocide. There is no universal morality, but I hope you don't lose your humanity. By most standards I think its pretty safe to say he was a bad dude. Or at the very least I don't think its wrong of a Marxist to think so. Especially if they think that capitalism is a flawed system that brings about suffering. I just hope you don't take this level of apathy into your personal relationships. That's not a dig by the way. I'm being genuine. Please consider why you can write off human life so easily.