gurugeorge
Arcane
Nope.Nope, even if they're about to get wiped out by a nearby supernova, the aliens are still in charge of all the causality leading to the situation. They created the engines that would lead to a genocide if not prevented.
Basically there's no way one can wheedle any sympathy for the aliens, they are totally at fault
First things first - there is no such thing as monolithic aliens* or monolithic humanity for that matter.
What we have is individuals (paladin, alien decision makers), singularly** making decision affecting a large number of non-participating parties - specifically selecting one bunch of those parties to be sacrificed so that the remaining bunch may live, rather than sacrificing others so that selves may live.
The decision-makers in either scenario (alien portal vs our portal) are at fault for whatever genocide occurs, not the paladins. There is no symmetry between the decision-makers' decisions and the paladins', the decision-makers bear all the blame.
These scenarios are quite different from your hypothetical accident.
Second, sacrificing someone else's life for your own is, other things equal, morally neutral.
No, it's evil. Opening a portal to an alien system so your own species can survive, without doing due diligence, is evil, and if as a result of technological limitations you can't do due diligence, then that's just too bad. Find another way.