Thomas Matthew Crooks wasn't a fan of Donald Trump years before attempting to assassinate the former Prez ... so says one of his former classmates.

Vincent Taormina -- who went to high school with the would-be assassin -- spoke with FOX News about Thomas' political beliefs leading up to Saturday's attack ... and he says TMC didn't like politicians, period. However, he apparently really recoiled at the thought of DT.

Vince says he and Thomas had gotten into a heated debate ahead of the 2016 presidential election -- and he describes the guy as smug and arrogant when discussing topics he was passionate about, which included politics apparently.

He claims Thomas scoffed at him for supporting Trump -- with Vince noting that he backed DT back then, citing his Hispanic heritage.

He continues ... "He would just talk, talk and act like he knew everything, especially politics related, and he would say it in a tone that was like, 'I'm better than you,' in a type of way."

Vince also somewhat pushed back on the notion Thomas was a loner who was actively bullied. On the contrary, he claims Thomas was part of a small friend group ... which had a reputation for making school shooting threats.

As Vincent recalled ... Thomas once allegedly didn't return to school for days after one threat was made against the school ... although TMC's involvement in that was never confirmed.

Thomas was shot and killed by Secret Service agents after opening fire at a Trump rally over the weekend. While his motive remains unknown, the authorities are investigating the shooting, which left Trump injured and a rally attendee dead.

This account about Thomas seems to fly in the face of what others have said about him -- namely, that he was a loner and a constant victim of bullying ... not to mention shy.