Karl Marx wanted that we should not only mask the pain of this heartless world with opium (a pain killer) but that we should change this heartless world itself.
He said the world is a valley of tears (vale of tears). And religion is its “halo” (circle of light indicating holiness). That means, religion gives us hope that this valley of tears is bearable because there will be justice in the afterlife.
But Marx wants justice in this life. So he asks us recognise that the world is full of exploitation. To throw away our illusions that this world is anything but a valley of tears. And he wants us to break our chains of exploitation and create a society based on equality where people will not need to create illusions to mask their pain and suffering. In other words he says, we don’t want pain killers. We want an end to the disease (i.e. exploitation).