Again, Sans mentioned that 'Sins crawling on your back.' Sans feeds off sins. The more sins you committed, the more powerful Sans is. If I remember correctly, killing someone who is going to destroy the world with absolute no choice, is actually NOT a sin. It is a necessary action for protecting someone with no absolute choice. Again, Kirby only kills if the threat is too great to be left alive to hurt since all of Kirby's foes are at least planetary to solar system threats and possibly universal threats who are trying to rule or destroying everything. It doesn't matter if Kirby kills, he doesn't kill in cold blood. He only kills if there is no other way against godlike beings like Magolor which is actually not a sin, making Sans' Karmic Retribution less powerful.
Edit: Also, the only real sin Kirby committed was just steal his rival's cake at the title opening of Kirby: Return to Dreamland. That's only one, but nothing completely major. Also, Sans in round one is going to hold back considering Kirby's necessary actions. However, let's say that Karmic still works should Kirby have no mercy which is possible. However, since neither Kirby nor Sans will take each other serious in round 1, Kirby will still win Via Inhale, ESP, Hypernova, Time Manipulation, and Mike Kirby. Round 2 is bloodlusted, so Kirby may have more trouble in round 2. Still, Kirby can fight through Karmic Retribution.
Round 1: Kirby via one punch, ESP, Inhale, Hypernova, Time Manipulation, Mike Kirby, or possibly Ice Kirby.
Round 2: This is closer since both are bloodlusted and speed is equalized. Kirby would still win 6/10 because of his ESP, Inhale, Hypernova, Time Manipulation, Mike Kirby.
Actually, Mike Kirby will be enough for round 1 because Kirby's singing talent is so horrible that everyone nearly dies. How is Sans going to teleport in different directions once Mike Kirby's singing hits Sans' ears? the only way he can do that is teleport further away enough, but that's still unlikely to happen in Round 1.