Why does he kill the little person?

Yes, that and he needed the gun.. And the little guy wouldn't give it to him.

I think the director may have wanted to draw our attention to the fact that the crims may have gone and provoked the wrong hombre.🐭

He needed a gun and didn't have the $300.00
Tried to talk him down to less, but no deal.
So, he shot him and got the gun.
The kind of bloke who does what it takes to get what he wants.
Case in point later when he shoots the cop.
He needed Ray and the cop was in his way.

It's the harbour towns that the grey metal ships call home.

I think he might have just paid him and left, but after seeing him throwing rocks at the dogs he decided just to kill him.

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http://tinyurl.com/k6hhn2d
Fear the man with nothing left to lose.

When possible, the rover tried to make fair trades, so I don't think his shooting the dwarf was out of character. In his initial pursuit of the stolen car, he was implicitly giving the thieves the opportunity to trade back the car they had taken for the truck they had abandoned. With the dwarf, the rover was prepared to make a fair trade. The dwarf was just dumb enough to hand the rover a box of loaded weapons and then be inflexible with his price. The rover offered the doctor money for her services because it seemed fair to him. The first town's general store owner had a policy similar to the rover's.

Yo, like a dog, bitch!

Late reply here but who cares 😉 I think it's a combination of Colin throwing rocks at the dog and calling Eric a f*k'in psycho and telling Eric that the guns are $300 US and if he doesn't have $300 he can go *beep* off... and that's when Eric blows him away. Eric is a ticking time bomb, it doesn't take much to set him off. 

"Ain't life grand!"