Even friendly aliens can cause risks for humanity.

There are two types of risks. That ETI (Extraterrestrial Intelligence) can cause. The first one is internal. The ideas of ETI are against the ideology of some religious people. And that thing can cause violent actions from those people. But if we think that we would someday face the ETI. That thing can cause many types of risks. 

The biggest risk is that the friendly aliens are telling lies. That means they are playing friendly but are hostile. 

And they will come to conquer our planet. But that risk is minimal. If we would get alien contact or confirmed alien contact that thing causes a big change in the human way think and seeing things. 

Cultural differences between humans are a huge thing. So what would you think about differences between humans and hypothetical Kardashev scale II or III civilizations? 

In real life, hypothetical contact between huge-scale civilizations can involve multiple dangers. There is the possibility that aliens are hostile. Or something goes wrong in the communication. In that case, the aliens will use too much power in communication to destroy the entire planet. 

In some other scenarios the power source of their craft, which can be like antimatter would accidentally be released into the atmosphere. That thing can blow the entire Earth. 

The environment and especially microbes on Earth are different than on an alien planet. So that means the microbes will eat those aliens that are landing on our planet. 

And those traveling aliens are the biggest risk that we can imagine. If aliens are traveling by using the fleet of giant O'Neill cylinders. That means they can be cosmic refugees. Or maybe one day in the future automatic alien probe is coming to our solar system. 

It is the possibility that there is an error in those systems. And that causes the alien spacecraft to impact Earth. 

But there is one another risk. If the alien craft uses a black hole as an energy source or communication platform there is a possibility. That something damages the core of the hollow ball where that black hole is. And the Earth will fall in that thing. 

Or there is the possibility that the aliens and humans are making contact. And something will interpret as a threat. Then another side will open fire. 

There is a possibility. That those hypothetical aliens do not think that some civilization has a lower technological level than they have. So those aliens expect that every other civilization is using wormholes as communication. And that can be risky business. Because if those aliens would try to create a wormhole on some other planet, that thing can cause very bad destruction. 

In some other versions, the aliens use tiny wormholes as signal transmit. So, aliens will start to create a wormhole to Earth that makes communication more comfortable. Then the huge energy load will impact Earth. 

Image: https://www.express.co.uk/news/science/1179496/alien-contact-end-times-seti-aliens-discovery-space-universe-news-stephen-hawking

Can multi-species civilization be possible in the universe?

When we are thinking about the situation that the alien race will meet another alien race there are two possibilities. The first possibility is that those alien species are starting diplomatic relations and the second possibility is that they will go to war. The thing is that the technically advanced civilizations that can travel to another solar system should realize that the war between civilizations would be deadly or at least causes extreme damage. 

The requirement for contact with aliens is interstellar travel. Without interstellar travel, contact is impossible on the physical level. But if the species is highly advanced it can create extremely powerful lasers that can cause damage even on an interstellar scale. The interstellar laser system can base on a mirror or a large group of mirror satellites. That is pointing their radiation to one large laser system. 

The advanced civilizations must have some kind of technology that can use as a weapon. The lasers that are planned to use to push solar sails to Alpha Centauri can turn into weapons. And that is the thing that causes criticism against that project The interstellar lasers would be far more powerful than those lasers that are planned to use to send solar sail to Alpha Centauri. 

Those systems are needed to protect civilizations against meteorites. And they can use laser or antimatter technology. 

This is the reason why the interstellar- or Kardashev-scale 2+ civilizations would probably choose communication and diplomacy. Things like Death Stars and antimatter weapons are the things that might help to tolerate each other when the interstellar civilizations are meeting. 

So the interstellar civilizations are probably not a threat to each other. Interstellar travel requires a certain type of behavior, and that means those aliens would remove aggressive behavior from their race. That thing can happen by using genetic engineering, where aggression and narcism are removed from their genomes. 

But as we see aggressive behavior and things like willing to destroy the other species depends on the culture. The thing is that the interstellar civilizations must be mature enough that they finish their wars and start to work for the goal that gives them the ability to travel to the stars. The highly advanced technology makes those species hard to destroy.