sniper enjoys the way the cat screams, solely because it adds to the fantasy. sniper does this a lot until he gains the balls to actually rape the blu spy, and he's surprisingly good at it too (having had a lot of practice)

He thinks it acts like it's better than everyone else just like Spy and is fancy and has a soft rich coat just like him and Sniper wants nothing more than to ruin that. Meanwhile the cat was just existing doing its thing. Perhaps the cat is also deaf due to the gene mixture of white with blue eyes so it didn't even see him coming. Very unspooklike but he figures Spy is so self-absorbed he wouldn't possibly think someone could sneak up on him so it still works in Sniper's mind.