Question: Spoiler: It's revealed that the first smaller predator was coming to bring a weapon to the humans to fight back against the bigger Predators. If this was the case, then why was it killing people when it first crash landed? It came across a completely different situation going on with soldiers taking out other humans, and it starts killing them, giving Quinn a chance to fight back and steal it's stuff. But why was it killing people to start with if it was bringing us a weapon to help?
Question: Why would the main character send the obviously dangerous and unstable alien tech to his house? I could see no other reason then to just simply get the plot to move along but it's obvious he doesn't have a consistent stable relationship with his son and wife - why send the tech there? And on top of that with no explanation to them as to what it is? On top of that he seemed like he was a respected member of the military - there was no reason for him to go against that and hide this information.
Answer: McKenna states that he assumed his estranged wife would just put the box with the rest of his stuff and leave it alone. He doesn't trust sending it to anyone else because they might open the box and have questions. As you suggest, it is indeed quite an egregious plot contrivance because why would his wife be any less prone to open his mail than anyone else? And in fact, Rory does open the box so the plan didn't even work. McKenna hides the tech from the military because he wants the credit for discovering alien life, and he also fears the government will try to cover up the discovery, which of course they do.
Question: If the small predator wants to help the humans by giving them the armour, then why does he start to fight and hunt them?
Answer: Predators are proud hunters and by giving humans armour they are giving themselves "a challenge." They are making the humans harder to kill to increase the pride from killing them.
With all due respect, this doesn't answer the question being asked. The first Predator specifically came to earth in order to provide technology that would allow humans to defend themselves against other Predator attacks. Its literal sole intention was to protect them, which is why the larger Predator comes after it. Not one single human that this Predator then goes on to kill is equipped with this armour, in fact some of the first people it kills are unarmed scientists.
Question: The movie Predator, is supposedly canon to only the first Predator film and never acknowledges the second film from even happening. Sort of a Superman Returns deal where it's a sequel to an earlier film, retconning the later films. This movie, however, acknowledges both of the first films but nothing about Predators is ever mentioned. But considering the premise of Predators, maybe just nobody in the film researching these aliens knows about the events of that film, however unlikely. So my question is this movie canon with all 3 previous films, or does it not count Predators?
Answer: Well, in the case of Predators, it was violent criminals, mercenaries and military being randomly kidnapped and dumped on an alien planet. From the perspective of humans, it would appear that the criminals and mercenaries had gone missing in a more mundane fashion or simply died. If the Predators were covert about it, the Predator researchers would not be suspicious at all. So it's entirely possible it's canon.
Answer: The second film is actually subtly acknowledged by Jake Busey playing a scientist named Keyes, implying that he is the son of Peter Keyes from the second film who was played by his father Gary.
Answer: The Predator had landed in the middle of a group of hostile Earth soldiers, who it judged would likely have just shot it immediately. It would have defended itself. And there's also the fact that it could just be for fun because, hey, old habits die hard.