In the second Predator movie the Elder Predator gave Lieutenant Mike Harrigan a pistol dating back to 1715. In Alien versus Predator the Elder Predator gave Lex, the last survivor of the Antarctica incident, a spear. In the 2018 Predator movie The Fugitive wanted to give Humanity a prototype weapon.
It doesn't make sense because when somebody kills a predator I always thought that others would appear and they would attack you but instead they reward you.
This question shows a fundamental misunderstanding of the Predators' culture. They aren't mindless killing machines; they're hunters with a strict sense of honour. They don't view the humans they hunt as "enemies", but as worthy opponents to test their skills against.
when somebody kills a predator I always thought that others would appear and they would attack you
That wouldn't be honourable. If a Predator takes on a human and the human wins, that means they've earned their survival. And in the first two cases (Predator 2 and Alien vs Predator), it means they've earned the right to a trophy. It's the Predator's way of honouring their skills.
In the case of the Fugitive, meanwhile, the fact that it wants to give the prototype weapon to humanity is the reason it's a fugitive in the first place: the other Predators disapprove of it giving away such powerful tech, and the Ultimate Predator is dispatched to kill it and retrieve the weapon.
