Well, first we need to remember that in Letter 131 Tolkien very specifically calls Sam the "chief hero" of LOTR, but he does also say that Sam has his faults too and is not wholly attractive to all readers.  Sam is somewhat narrow-minded with regard to certain issues and, unlike Frodo, has to learn pity the hard way.  But he does overcome those limitations and that is what makes him a character who develops and changes, and grows into the role of hero.

The parts you quote are from the earlier stages of the journey when Sam's distrust of Gollum is at its highest.  And yes, there he doesn't look so magnanimous to say the least in his attitudes towards Slinker and Stinker!  But you do not cite the most crucial passage in which Sam has learned the lesson of pity:  his descision on Mount Doom not to kill Gollum after his last desperate attack on Frodo:

It would be just to slay this treacherous, muderous creature, and just many times deserved; and it also seemed the safe thing to do.  But deep in his heart there was something that restrained him: he could not strike this thing lying in the dust, forlorn, ruinous, utterly wretched.

This is a pivotal change in Sam who earlier DID have murderous thoughts brought on by the intesity of his dedication to Frodo.  But then he learns pity, in part from having carried the Ring briefly, but more because he does in way see through Frodo's eyes, though he can't express what he feels. Sam's main motivation is to stick by Frodo and help his master at all costs, the role of the batman in the World War I battlefield, so he is earlier so dogged in his love that he does become rigid in his views on Gollum.  We also see glimpses of his change in his thoughts on the fallen Harad soldier (lines that the film gave to Faramir to make him more likeable; Sam also cannot articulate his thoughts, so the narrator does it for us).  He's less saintly than Frodo, so his journey is coming to see that pity can be felt for a creature like Gollum who means to do them harm.

The points you raise are valid because Sam is not a wholly noble character in all aspects.  Yet it is his growth and his discovery of his own ability to be like Frodo in a way at the end of all things that makes him a strong character.  Aragorn and Frodo do not have to overcome as many obstactles in their own nature and views.

  Nasmith, Eärendil the Mariner