Chief Benja believed that one should not withhold trust from someone just because they are from another tribe or another land. However, there is a distinction in this trust. Distrust will only make things worse for everyone.
This is enough to motivate Raya to take defensive action but it is left unclear if Namaari was going to intentionally fire on Sisu out of desperation or was so emotionally overwhelmed by the presence of a dragon that she'd pull the trigger by accident.
Long ago, the people of the mystical land of Kumandra were united and protected by dragons. Raya and the Last Dragon is the 59th entry in the Disney Animated Canon.