The question comes down to this: is a life full of suffering, torture, violence, and misery better than no life at all? For me, the answer is no.
On the other side, many wild herbivores and prey animals live lives in an almost perpetual state of alertness and fear, just a moment away from being viciously torn apart and eaten alive.
According to the National Council for Animal Protection, over 99% of beef farmed for meat comes from cows that live in horrific factory farm environments.
This doesn't necessarily clash with what you said, but I feel it bares pointing out that less than 1% of feedstock is NOT kept in conditions that perpetuate a state of fear and alertness (basically all factory farming is like that).
"Even if, on net, the humans in the alien world suffer, is oblivion really the preferable alternative to net suffering? Especially if you could alleviate that suffering through action?"