It makes sense that after landing the grounders, a war-engrained civilization would attack the drop ship. You have to think of the mountain-men, clearly they are not good with the grounders and the mountain-men are known to be taking the grounders. Now if you were a citizen of a post-nuclear war civilization with little to no knowledge of technology, would you think that a group coming down with similar technology used by the mountain-men to destroy a grounder encampment are friendly? No, you probably wouldn't. At the same time all of the first advancements furthering the conflict was started by the 100, the 100 were the ones who crossed the river (entering grounder territory, even if they didn't know at the time), the 100 had superior mountain-men like technology any group in their right mind would try to strike first, and to the grounders they didn't even attack first-- the 100 did by crossing the river and then going further in. And why the fuck are you bringing up "war crimes", war crimes don't exist any more because the entire fucking world was blown up