It would not be unreasonable to carry out a preemptive strike against a species that is extremely hostile to itself, and would likely carry that potential hostility into inter-species contact. It's simply self-preservation, something that any technologically-advanced species would have.
Yeah, but one of the cool things about humanity (and probably other species if they also exist and we aren't in the universe of
Blindsight) is that we don't always have to do the most logical thing. I mean logically I should be attempting to exterminate and genocide every other person on the face of the planet who ever stands a chance of harming my interests, but that doesn't mean I am going to. Honestly I don't really understand why people always seem to assume that the idea of genociding another species is something that anybody that was similar enough to us to be able to communicate would do. If anything the backlash to the idea from the alien hippies would be enormous.
I mean look at it this way. We have had the technology to cause somewhere around 40 different species of mosquitos to go extinct, right now, for as little as a couple billion dollars and 3-5 years of work. Doing that would save over 600,000 people
every year, would virtually eliminate a huge number of extremely deadly diseases like malaria and yellow fever, and would repay its entire cost in only a handful of years due to reducing medical costs globally. We have expert scientists who have done study after study showing that such an elimination wouldn't even cause any significant harms to the environment, and even endorsing such a plan.
And you know what? Nobody wants to do it. At this point there have been exactly two pilot studies, both of which were cut short (one by a civil war, one by a false mass hysteria). Nobody else wants to do it, because governments and funders are (rightfully) concerned about the huge backlash that any such "genocide" attempt would provoke. We've spent the last 40 years telling people over and over they need to do everything they can to save the natural world, and now you want to purposely make a species go extinct? And that's just a lowly bunch of mosquitos, can you imagine the backlash if you were trying to extinguish a species that could reason and communicate at a high level?
I don't know about you, but I've never seen or heard of someone who was willing to attempt to genocide anything and was considered "normal" in any way, and honestly I don't see any reason why an alien species would be different. One of the neat things about sentience is that it lets you override the "kill everything that is different" urge, and see the benefits that come from communication and working together.
Evolution endowed us with a natural tendency to be kind to our genetic relations but to be xenophobic, suspicious, and aggressive toward people in other tribes. As our brains become better equipped to reason abstractly in such tasks as lumping dogs and rabbits together into the category "mammal," so too have we improved in our capacity to lump blacks and whites, men and women, straights and gays into the category "human."