Basically, Khuswant Singh wants to convey the irony of one's beliefs in The Mark Of Vishnu.
In the story, Gunga Ram is a devotee of Vishnu. Vishnu is the preserver and protector of creation. For Gunga Ram, all living things are to be revered and cherished, even dangerous animals.
To him, all life was sacred, even if it was of a serpent or scorpion or centipede.
For the most part, he is an object of derision to the boys in the household. Above all, Gunga Ram's great regard for the Kala Nag (cobra) creates conflict between him and his master's children. It is obvious that Gunga Ram derives a sense of righteous dignity (in his position as a lowly servant) through his devoted defense of such dangerous creatures. Meanwhile, the boys are more concerned about displaying clear evidences of their masculine mastery over such treacherous wild enemies.
In the story, science is pitted against superstition. Both are sorely tested, but science wins in the end. The hapless Gunga Ram is attacked and brutally poisoned by the Kala Nag even as he bows his head in obeisance to the cruel god.
Gunga Ram collapsed with his hands covering his face. He groaned in agony. The poison blinded him instantly. Within a few minutes he turned pale and blue and froth appeared in his mouth. On his forehead were little drops of blood. These the teacher wiped with his handkerchief. Underneath was the V-mark where the Kala Nag had dug his fangs.
In describing the manner of his death, Khuswant Singh is highlighting the irony of Gunga Ram's implicit trust in his beliefs. Far from ensuring him long life, his blind worship has caused him to endure a cruel death. Alas, Vishnu, the one god who is supposed to tip the balance of the forces of good and evil in favor of the good, has chosen not to preserve the life of his seemingly devout supplicant, Gunga Ram.