Kes talked to maybe two or three of them. All of them piloting those bioships. I doubt a soldier flying a single person ship is going to be representative of the whole species. That's like reading the mind of an air force pilot in combat thinking, "Kill all those mfers!" and drawing the conclusion they're going to commit genocide.
Hate to say it, but Voyager was the aggressor with 8472. They tried tractoring their ship, beaming it away, and boarded it without permission. Then they were later seen collaborating with their enemy. Helped them design a biological weapon of mass destruction oh and handed one of their people over to a race of savages that probably disemboweled the fellow. So yeah... some aggressive tendencies towards Voyager are probably to be expected from the people doing the fighting.