The relationship between Sniper and Spy is a well-crafted one. Both are assassins who fulfil the same role and pride themselves on professionalism. It's their definition of "professional" where they diverge.
To Sniper, professionalism is defined by detachment and efficiency. Taking a life is serious business, and so the job is prioritized above everything, including basic hygiene and bathroom breaks.
"Snipers don't muck about... we just take the shot."
To Spy, professionalism is defined by dignity and artistry. Taking a life is serious business, and so it must be handled with grace, even when fighting lunatics in the middle of a desert.
"Six unstabbed backs... I do love a blank canvas!"
Sniper considers Spy unprofessional and cowardly due to his pompous demeanour and underhanded tactics. To find artistry in murder is psychopathic, to engage in trickery, dishonourable. Spy is an arrogant snake that gives proper assassins like Sniper a bad name.
"Ya bloody backstabbing fraud!"
Spy considers Sniper unprofessional and cowardly due to his oafish demeanour and impersonal tactics. To murder without feeling is psychopathic, to attack only from a distance, dishonourable. Sniper is a tactless brute that gives proper assassins like Spy a bad name.
"You disgust me, filthy jar man!"