That could be true.
Judging by history, that's not a very good bet. Peace sort of works among humans, in part because we are so close genetically. But Pre-WWI Europe showed that you can't engineer peace even by inter-marrying rulers.
Stepping away from humans a small amount, even today people regularly eat primates of various sorts (i.e. "bush meat").
Why do you think humans would ever make peace with alien cultures?
Great question. Maybe we need interstellar mutually-assured destruction. Seems to be the only thing, short of dominance, that has worked to preserve peace(-ish).
It seems unlikely to me that when 2 alien cultures meet, that their technology levels are so similar that MAD is possible.
Whilst I doubt that (all of?) our viruses would have any effect on them (as they require our own cells to reproduce - aliens are unlikely to have a similar makeup), but things like funguses and bacteria could be very very bad for them.
Random bacteria and fungii that are introduced to humans don't kill us all off. Bacteria especially are everywhere, it stands to reason that over all of the exploring, investigating and just plain spreading that we do, we'd encounter some bacteria that are new to a population. If those had any great chance of killing us off, shouldn't at least local examples exist of that happening?
Instead I think what we see are the organisms that affect us are those that coevolve with either us or with related species. We're killed by the things we spend a lot of time with, not random out-of-context bacteria that's never seen a mammal before.
If that's true of us, seems hard to say why it wouldn't be true of aliens, no?
We're mostly killed by things that have never been in humans before because we haven't developed defenses. Sure, most of those things don't kill us, but you really only need the right one to come along and basically wipe us out. Part of the reasons we're ultimately able to develop a defense is because it is Earth life and we've seen all the tricks. Any alien microbe that is able to establish itself inside our bodies might not have to worry about any of our defenses.
I think you've really got the wrong model. Think of the introduction of non-native species to various ecosystems. A lot of the time, the non-native species either can't hack it, or they find an equilibrium. But some of the time you get kudzu or eurasian watermilfoil.
In any case, the point is largely moot. Any civilization we are likely to meet in the next thousands of years will necessarily have to visit us and if they can do that, they can just lob a couple space rocks at Earth and wipe us out from a distance.
We have examples of exactly this! It was very bad: https://en.wikipedia.org/wiki/Native_American_disease_and_ep...
A bacterium that hasn't seem humans before is pretty unlikely to be able to do anything to us. One that hasn't even been in mammals before is even less likely, etc.
What if organisms could choose other ways of allying and joining together? What if, say, you had the strong, dominant, ambitious sub-population of a race trying to ally with other strong, dominant, ambitious life forms without thought to race? What if the organisms in the universe which favored peace decided to seek each other out and unite for a common cause?