Well, in the absence of any lore from the developer, I am happy to take this and go with it.
Why do you think the Tenno are being dicks? Because we keep invading their sh*t and f**king it up?
A few of the things The Stalker says and the fact some of the major NPCs call the Tenno "The Betrayers" strongly suggests that they have a background in superdickery.
Mostly, though, it's the actual missions. Aside from the Exterminate missions - "Tenno, we need to send a message to the Grineer.
Kill fucking everyone" - there's one Assassination mission that made me feel downright
sleazy.
"Tenno, the target of this assassination is a brilliant geneticist making great strides in the gene repair of the Grineer. Left to continue his work, he could potentially reverse centuries of genetic damage done from overuse of cloning technology."
"Oh! So, we're going to rescue this guy, huh?"
"No, Tenno."
"... Capture?"
"Tenno, he must not be allowed to repair the genetic degradation of the Grineer. We need these guys having some super gross stillborn babies. For reasons."
"I, uh... I guess that makes sense. Um, Lotus?"
"Yes, Tenno?"
"... you'd tell me if you were a Nazi, right?"
"Of course, Tenno. Now hurry up and murder this doctor before he can save his race."
"Kaaaaay."