Killer Sans was made by Tumblr user Rahafwabas.
In a genocide route, Sans joins Chara on their genocide. There are 3 interpretations of these timelines:
#1: He killed Chara and continued on his genocide.
#2: He joined Chara and made a partnered genocide.
#3: He joined Chara in his genocide but later decided to kill them and continue his genocide.
Note: #2 and #3 are the most common routes.
He looks much like Classic Sans, but with a red target-like soul displaying, and liquid hate pouring out of his eyes.
Maniac,and dislikes sparing (i think)
Real Knife: Causes 99 damage.
Sharp Bones: Usually has high attack but deadly when attacked at the sharp point.
Way More HP: Because of him having determination like Chara, he has gained way more health points than most other Sans, though it's unknown how much exactly.
Reset: He has the power to reset the timeline, but if he does this his stats will be reset, making him LV 1 again but still with his twisted mind, though he can save which makes up for his reset.
Note: Killer!Sans has no Gaster Blasters... but he has one giant goop-spilling Gaster Blaster though It's full power is unknown, he uses it either when he's bored of his opponent or wants to end the fight very quickly
Killer!Sans loves to mess around with him. He is one of the only people who Nightmare talks with normally. He's the leader of the Nightmare Team in which Killer!Sans is included and is a valuable member. Nightmare keeps check of Killer that he remains in his dark state by reverting his soul back to its emotionless state from time to time, though Nightmare has a big rival when it comes to the soul of Killer, which is Color!Sans.
Genocide partner or purely hated rival. It depends on the user.
Another version of Sans who used the six souls in the genocide route. He is a dear friend to Killer even though he gets annoyed sometimes by him. He tries to revert Killer back to his normal good self, but Nightmare keeps check of Killer to stay emotionless.