Killer! Sans

Killer!Sans's personality is characterized by a deranged mind, a desire to kill or hurt others because it gives him pleasure. 1
Killer!Sans spends most of his time in his "second" state. During this state, he's unpredictable, plays on his victims' emotions, and is also extremely cruel: he calmly kills others without feeling guilty. A black liquid begins to flow from his eyes, and his pupils disappear.
Occasionally, Killer switches to the first state: during this state, Killer can feel some kind of emotion, is calm and predictable, and his “tears” disappear. 1
In the third state, Killer is enraged and attacks everyone. Sometimes, Killer begins to feel guilty, but consoles himself with the thought that others will forget everything. Sometimes, Killer even remembers Papyrus's death, for which he blames himself. Also, a part of him doesn't want any of this and longs for his friends and family to return.

a few years ago:
At the end of the Genocide Route, Chara bribe Sans, telling him there's no point in faking a smile and resisting, and offering him a deal. A broken Sans agrees, and Chara possesses him. After the reset, Sans tries to pretend he doesn't care about others, as they'll forget everything, but killing Papyrus leaves him with a strong sense of guilt. After several Genocide Routes, Sans's soul merges with Chara's, making him colder and more indifferent.

Killer Sans's appearance from the alternate history Undertale Something New, created by rahafwabas, includes:
a dark blue zippered jacket with a white fluffy hood;
black turtleneck;

white shorts with black stripes;
blue boots with white socks.
Sans's eyes are empty, with DT threads sticking out of them, now black. The character's face changes depending on his state of mind.