I don't remember if you said you'd watched REBELS or not, but in the show, there were SEVERAL times that Ezra flirted with the Dark Side. He was able to open a Sith holocron, he tapped into the Dark Side to get creatures to attack their enemies, he went with Maul as he flirted with the idea of being his apprentice, all of which gave Kanan some extremely worrying vibes over the course of the show.
He basically had a 'how bad can it be' attitude from time to time, if he thought the Dark Side could help his friends.