Alejandro is a villain because... he antagonized the first season by manipulating people. He hedged numerous eliminations and drove Zoey to insanity, made Nathaniel lose his reputation, and even bullied Mike for his disorder. Alejandro used many girls to get his way, such as Beth, Bridgette, Heather, Zoey, Sadie, Katie, and Dakota.
Mel is a villain because... she entered the game and tore apart a relationship between Courtney and Duncan. She stepped on manypeople'sfingers to do so. Even after she won Duncan over, she continued being a bully to Courtney and eliminated her new boyfriend.
Noah was a villain because... he could possibly be tied with the destruction of the Total Drama Jumbo Jet that fell apart in Planes, Trains, And Hot Air Mobiles. He also is generally a snarky and pessimistic person.
Dakota flirted with Alejandro in hopes of gaining a fanbase who shipped them. "Alekota". This angered Heather, who clashed with her out of jealousy. Alejandro blindsided her just to see if Heather cared for him.
Shin and Leshawna were considered the most heroic villains on the team. They'd be threats after the merge, as claimed by Heather, Jo, and Alejandro. Shin accidentally betrayed the villains and won the challenge for the heroes.