Though he was a member of Dumbledore’s Army (a group who made it their mission in life to resist Umbridge and learn defensive spells), Zacharias Smith was a nasty individual full of spite.
He was an irritant from the moment he appeared in The Order of the Phoenix. During the DA’s first meeting in the Hogshead, he sneered at Harry’s story about Voldemort’s return and was sceptical about how Cedric had died. Zacharias continued to be the most unpleasant member of the DA and was often critical of Harry’s teaching methods.
To top it all off, Zacharias became a Quidditch commenter in The Half-Blood Prince and took the opportunity to verbally abuse the players. He was particularly nasty to Ron and Ginny and suggested they were only on the team because Harry liked them.
Whilst it would be wrong to describe Zacharias as evil, his constant taunting and his mean-spirited personality made him one of the most insufferable individuals in the Harry Potter series.