Everyone thinks the marines are dumbasses, the marines are usually the last to find out they are fight aliens instead of a riot or some other coughout excuse.
In the first alien movie, they had an APC, a dropship with a

load of missles, but the stupid co-pilot let an alien in, and now the marines are stranded.
Marines would own, if everyone knew they were fighting aliens, even the pilots to drop ships, the aliens will go where the meat is. Their never really prepared, because the guys in the company want an alien, but dont tell the ppl down there fighting to get the alien, that their most likely gunna die, even if they succeed in capturing an alien.
The people in the first predator movie were sent to see wtf happened in the jungle, and were expecting to kill the terrorist's, instead of a very skilled predator.
Aliens arent smart, their 'intelligence' is basically instict, like dogs have the instict to hump eachother.
Aliens usually win in their element, the hive atmosphere, they can hide practically anywhere very effectivley, even enought to where a predator wont imeidiatly see them.
The predators are very smart.
But for some reason, their equipment should be acid proof to a certain degree, their blood has an agent that neutralizes the acid, they should be able to find a way to make an alloy that can withstand moderate amounts of acid.
The predators dont hunt 'oomans' (softmeat) with any shoulder burners, even if they are marines... theyll only use those for the aliens, witch is why the preds had shoulder burners in the avp movie.
The yautja dont really have any laws, just guidlines and creeds, and like every intelligent race, theres always the ppl who ignore the rules and such.