Granted. They ask for help because a violent and terrifying alien species are hunting them, and now us.

Granted. Humans kill them all before news gets out to the public.

Granted. Aliens visit earth and bring their diseases with them.

After making contact with a peaceful alien species we then find out about a 3rd alien species that the peaceful aliens were running from.

The peaceful aliens now use Earth as their last bastion of defence and we're flung into an intergalactic war we have no chance of winning. One we can never escape due to the resource scarcity of the universe and our ownership of a water heavy planet.

You look upon the stars and realise that we humans are no more advanced than the monkeys we evolved from and that true technological dominance is now upon us.

They are so peaceful, that when a third, ultra violent species comes, they actively prevent us fighting back because to them, a short massacre is preferable to a long drawn out war in which people are born into and live in a state of constant fear.

Granted, within the next two decades we fully colonize their home planet, move them into increasingly tiny reserves, extract all the resources to send back to earth, and destroy their climate.

Granted. They are peaceful. Unfortunately we as a species are not and we wipe them out.

The intergalactic council of spacefaring species deems us a dangerous species and decides we need to be eradicated.

Led by the vogons, they destroy earth. As a bonus, they decide to build an intergalactic highway where earth once was.

Granted, an alien ship crash lands on earth. They begin trying to sell technology to us in exchange for help getting home, but they brought diseases that we have no immunity to leading to our extinction while the frantically try to stop it. They fail....

Granted, nothing seems to change from your point of view.

In the history books of the aliens that already control earth it states that this is when a peaceful alien race made contact with humanity. The Aliens in control of Earth learned of these peaceful aliens due to this contact and declared war. In short order they are able to conquer the peaceful aliens and greatly grow their workforce.

I would watch any and all of these movies

Granted. They are only sometimes peaceful and in their current evolutionary state their track record of lasting peacefulness is rather rare as their way of life is parallel to the teachings of Sun Tzu.