When we start colonizing the cosmos, are wars over habitable worlds and resources inevitable

Well only if those habitable worlds have indigenous inhabitants.

But not to worry,
If they are empty of life we'll soon find something to go to war over.
You know, civil war, war with earth, war with another colony.

Earthlings are good at going to war, especially if we know we are in the right.

No. As we come closer to that time, wars will more and more be fought with finances and laws and less and less with soldiers and guns. In other words, the privately owned company that can get a ship to planet blank will be the one who makes the mining claim.
With a global economy, national pride will be insignificant and meaningless. Why would we squabble over which country gets the right to mine pluto, if the benefit will be felt world wide.

Of course not - not least because we have absolutely (and I really mean absolutely) no evidence of intelligent life anywhere else in the universe, or of life of any kind outwith the solar sysytem. Nor do we have clear evidence of any kind of life anywhere other than on earth.

Another point you might like to bear in mind is how very far apart stars are.

yes I believe so, because no species survives without nothing, and since all existing speicies have the instinct of survival, or else they wouldn´t exist, a space war would be inevitable and we better be prepare d for this when the time comes to colonize other star systems

I belive so, it's not just humans, that have this violent streak but yes if we go into space and find stuff that's worth anything people will fight over the resources.

We won't be. Manned interstellar travel is about as likely as E.T. coming down and flying a bicycle.