8472 defended themselves from the Borg when attacked and had the audacity to counter attack. Their first encounter with Voyager, they attempted to tractor one of their ships, beam it away, then boarded it. Yeah, I'd consider all that hostile too and they to be in collusion with the Borg. Especially since they were actively protecting Voyager the second time they encountered them.
8472 was later shown to be quite reasonable when they're not being invaded. The whole "destroy the galaxy" thing is just Kes's intuition, which could be inaccurate. "The weak shall perish" could just be a battle cry for example, certainly I'm sure some of our generals said we should bomb Germany off the face of the Earth or purge all the Nazi's during WW2, but we didn't quite do it.
Picard would have actually tried TALKING to them before just randomly deciding they're a threat because the Borg might have a meal ticket across their space in all this. To say nothing of Picard would stop and consider two things, other species the Borg will attack after this war(like the guy in Hope and Fear's race) and the opportunity to bring the Collective to it's knees in negotiating with the 8472.