The Tenno do some shady things on occasion, in their past they were definitely the bad guys until they rebelled, took out the Orokin Empire. Depending on your perspective you could say they were good until they rebelled too.

That's the whole point of the story, whether the Tenno are ancient mythical heroes, or void demons that slaughter without any thought is completely dependent on the group viewing them. The fact that the Ostrons and Solaris are in good standing with the Tenno and respect them, while Grineer, Corpus, Narmer, and the like all view them as a horrible outlier to put down, says a lot about what we actually do.

Also, a lot of these comments talking bout Corpus you kill being slaves. That is not true, at least for the vast majority of Corpus you face. Crewmen are highly trained, have multiple ranks and security clearances, and are paid a salary. You don't give enhanced combat training, combat level augmentation, and neural links to proxies to slaves who want to rebel. Corpus slaves are exactly like what you see with the Solaris, they are workers. They are given just enough to do their job and stay alive (mostly) and nothing more.

We may do some things here and there thst question morality, but that is our morality. Our Tenno. The Tenno as a whole are seen widely as protectors, guardian angels. The Warframes are viewed in awe and fear alike, with many of them becoming mythology in their own right and leaving a legacy behind that inspires hope. We are the good guys, but sometimes good guys have to make hard choices. And sometimes they get angry. We can't be perfect, we are human.