Here’s a Wikipedia article that explains what a “Tankie” is.
TL;DR - It’s a slur to refer to authoritarian communists, originally used to speak out against people who supported using tanks to suppress dissidents.
Generally, anyone who supports aggression or violence to silence or oppress people are referred to as Tankies.
By the way, I took these three screenshots in order during my gameplay. While reviewing them later, I thought they were hilarious out of context. Enjoy!