OMG 'The Mission' - what even WAS that ending?!

Okay, so I finally bit the bullet and watched 'The Mission' (1986). I knew absolutely NOTHING going in, just that it was old and 'important.' No idea who directed it, honestly. The plot is basically a Jesuit priest goes to South America to convert some indigenous people, and then a slave hunter dude joins him after having a serious redemption arc. Spain sells the land or something, and then things get REALLY messy. First off, Jeremy Irons is fantastic as Father Gabriel. So serene and...well, priestly! And Robert De Niro as Rodrigo Mendoza? Damn. That scene where he's dragging his armor up the waterfall as penance legit gave me chills. The music, holy moly, the music! Ennio Morricone, right? It was so powerful, it almost felt like another character in the movie. But that ending? Seriously, what was the POINT of all that? I get it, colonialism is bad and violence is bad, but everyone just dies! It's so bleak! I was totally invested in the mission's success, even though I knew in the back of my head something horrible was gonna happen (because, you know, history). Seeing those kids get gunned down...ugh. It's hard to watch. And what was up with that Cardinal guy trying to justify everything politically? Talk about a tough moral choice. No easy answers in THAT situation. Overall, I think it's a really good movie, even if it left me feeling completely bummed out and a little confused, lol. I'm still processing everything. Definitely worth watching, though. Someone needs to explain that ending to me, seriously. Is it just meant to be depressing? Maybe I missed something. Anyone have any interpretations?