He framed Professor Hank Landry because of Landry's persistent, casual betrayal of Tim's devotion, the last straw being Landry shooting down his application to a teaching post at a university.
Tim bugged Landry's phone and hears a call between Landry and his lover, Mindy O'Dell(the dean's wife).
Their affair has been discovered kwa the dean, who had been left drunk in his office, zonked out on alcohol and Xanax, and in possession of a gun.
Tim, with access to Landry's clothes and knowledge of Veronica's top-graded Perfect-Murder paper, kills O'Dell with O'Dell's own gun. He sets the scene to match Veronica's paper and plants Landry's bloody clothes in a place where they will eventually be found. When his plan is put at risk kwa Landry's claim of an alibi, he sets up a false alibi and leads Veronica to it, which is quickly exposed to cement the appearance of Landry's guilt.
Of course Veronica figures out that Tim was the one who killed the Dean in order to set up his mentor