Disclaimer, just my two cents with how I understand this stuff, which is pretty surface level, all things considered.

The alienation described by Marx and Engels comes from the Hegelian master/slave dynamic within a capitalist context of wages and profit-seeking, where the value of labor is divorced from the product of the labor in such a way that the maker/bondsman/worker does not receive the compensation his creation demands (the value of his labor). In this way, class-consciousness of labor is a collectivized equivalent to the self-actualized fulfillment that Hegel describes. Both outcomes result in the destruction of the position of the Master (Capitalist) in the social hierarchy.

Put broadly, whether you're talking philosophical or economical, the Slave exists just fine with or without the Master (as the Slave would produce for his own needs, even if the Master did not exist to demand production), while the Master cannot exist without the Slave beneath him in the hierarchy, no matter what the Master does otherwise. Slaves do not need Masters, but Masters need Slaves to observe their superior position in hierarchy. In a more pithy way, for the Master, Hell is Other People, and their only two options are to perpetuate the hierarchy forever, or witness the destruction of the self as the Master, and lower themselves to equal footing with those they once demanded reify their superiority. Obviously, that's an incredibly difficult thing for basically anyone in that position to accept without being forced to do so.