It could be a 'sorceror's apprentice' situation: Millenia ago somebody instructed a self replicating, learning robot to retrieve some gifts for his wife's anniversary and then promptly died. Now there's a planet deep in space with mountains of jewelry, dead flowers, chocolate boxes... The robots descend on a new planet, severely interupting commerce as they steal gifts and natural resources for replication. Then they leave to deliver it home.

They could be police bots, designed by a long dead race to prevent intestellar warfare. Ships patrol the galaxies, if they happen to observe hostilities, in comes the robot armada. As long as you don't happen to be caught, you can war all you want of course.

Maybe the robots are just searching for understanding about where they came from, who built them. They have no interest in expansion, but do insist on access to worlds for their archeological digs (even if this means you need to move your cities for them). Something interesting could occur if they succeed.

It could be a single, moon sized robot, travelling from planet to planet to trade for or take by force supplies. It doen't conquer or settle or anything, it just floats through space demanding resources and then moving on.

The robots could be friendly, scientific minded beings voraciously researching all they can. They seem great, if you're willing to help provide resources for their work, they'll share research or tireless automatons. If you look into what they're doing, the big project seems to be a galactic sized particle accelerator. If your scientists look into it...hmm...wait, how much energy is going to occur in that collision? No, you don't really think that could cause a vacuum collapse, do you?

You should look on difficulties and advantages of living creatures, and difficulties and advantages of "less defecting creatures".

Most urgent need of living organic creatures is solved by nourishing on living creatures, and as such, their enviroment is rather stable.

This- the enviromental and regenerating difficulty of robots, and exhousted distribution of any possible vital resource upon all robots, would perhaps couse them very hostile towards any source that may benefit this intense issue of theirs. But rather than that, they should yield towards becoming organic creatures the most, since they would want the organic creature's system of regeneration and circular enviroment. They would speculate with samples of Fungus, Florus and Faunus, and they would be respected by organic creatures since they would be considered by them living rather than acting. The player might aid robots, since they would realy be souls/living creations, since this is the basic uncatchable base in universe, not the physical matter.

What comes to my mind is some sort of robot queen standing half deep in a pool of red blood during load screen.

And this is what I call "player raping story that you cannot aford to screw up on him and force bad" (I did not finish Risen 1 game since I was supposed to keep the poor Titan imprisoned, I had the same intensions as inquisitor Medoza that I had to slay... also, in Risen 2, my hero, when his ship got stolen by betrayer, he said "My ship, My crew, My Patty", and I hated Patty and I was in love with Chani, I was always taking her to accompany me, and when a guy come to me saying "Leave Chani alone" and my stupid hero responded without choice of mine "Calm down, Chani is not my type", I felt like crashing the screen)