Sans's personality is very erratic and he could snap at any moment, while still maintaining most of his former intelligence. The only person that manages to calm him down during a breakdown is Rose, but she may not always be there. Sans has the thought that everyone hates him, and that his only purpose is to exterminate the Human race from the Multiverse. To do so he will go through any lengths necessary, no matter how long it may take or how impossible it may seem.