The flip side of this idea is "cosmic rescue missions" (term coined by David Pearce), which refers to the hypothetical scenario in which human civilization help to reduce the suffering of sentient extraterrestrials (in the original context, it referred to the use of technology to abolish suffering). Of course, this is more relevant for simple animal-like aliens and less so for advanced civilizations, which would presumably have already either implemented a similar technology or decided to reject such technology. Brian Tomasik argues that cosmic rescue missions are unlikely.

Also, there's an argument that humanity conquering aliens civs would only be considered bad if you assume that either (1) we have non-universalist-consequentialist reasons to believe that preventing alien civilizations from existing is bad, or (2) the alien civilization would produce greater universalist-consequentialist value than human civilizations with the same resources. If (2) is the case, then humanity should actually be willing to sacrifice itself to let the aliens take over (like in the "utility monster" thought experiment), assuming that universalist consequentialism is true. If neither (1) nor (2) holds, then human civilization would have greater value than ET civilization. Seth Baum's paper on universalist ethics and alien encounters goes into greater detail.