Single word that means "to look down on others"?

You can consider condescend (to sb). If you condescend to somebody, you show feelings of superiority and you can even be patronizing toward that person. So, you actually show it in your actions and this can be called a condescending behavior.

to treat someone as if you are more important or more intelligent than them

_{http://dictionary.cambridge.org/dictionary/british/condescend-to-sb}

Further details from vocabulary.com:

If you are being condescending, you are looking down on someone. A 10-year-old who says to his sibling, "What do you know? You're only a 6-year-old!" is being condescending.

The Latin prefix con- means "with," and the Latin word for descend means "down," so the word condescending probably developed to describe someone who looked down on others. Condescending behavior is, not surprisingly, itself looked down upon. It's usually intended to make people feel bad about not knowing or having something, and it very often works.

Note: There is the pejorative term condescending prick also.

Related:
What does 'condescending' exactly mean?

• Disdain
• Scorn

Both mean to look on with contempt (or, in noun form, describe the act of looking on with contempt). (And there is the archaic contemn ) Both also imply a public, unashamed display of contempt (scorn even more so than disdain, I think).

He scorned their banal condolences; they were beneath his dignity.

We disdained him as a traitor and a coward.