Especially if his skewed view of superheroes from his personal loss has led him to see them a destructive force in the world, which isn’t any stretch from his agenda in Civil War just evolved into a broader viewpoint, and specifically Sam and Bucky are both very connected to Captain America and Sam is probably directly replacing him.
I think this idea of Zemo regarding ALL enhanced people (not just the Avengers) as dangerous was already evident in CW. That's why he killed the other Winter Soldiers and then joked to Bucky that "You really thought I wanted more of you?".