If you hate your siblings, it would be great for traumatizing them for life.

santa is in his own globe controlled by krampus in his dark and dingy dungeon that we see at the end of this film
Lee's Daniel's' THe Butler'

I don't think it'd ruin Santa. I mean, if KrampusSanta's punishment-dealing counterpartis real, why shouldn't Santa be real? The movie doesn't say Santa doesn't exist, and even the grandma says she believes. He just doesn't come that year.
Some of the bratty cousins mock the boy for believing, but they're jerks.
There is no objective reality and that's
Sucker Punch

It's not a kids movie so it shouldn't be a problem.

IF you think the ending as a 'good' ending, you COULD think that it was Santa saves them from Krampus once they were redeemed by Max's decision (and the family's constant sacrifice), and Krampus is only allowed to watch them now though like home, you never SEE Santa or St Nick just an idea. I thought about it once and told my kid and said it this way. If Santa is a force you normally do NOT see on Christmas Eve Night, but you could see Krampus, how can we not imagine it was Santa that saved them from Krampus? and not Krampus that saved them, despite Max confronting him saying/thinking how Krampus could 'put things back'. If Krampus is the shadow of St Nickolaus, who's to say it wasn't the 'non-shadow' (or other side of Krampus aka St Nick) that redeemed them. We just don't see him. We HAVE to remember it was Max's LETTER TO SANTA ripped up as his belief was shattered that started the whole series of events. Letters to Santa go somewhere. But his went somewhere else. (Where do these stairs go? They go UP. Okay, you first.) His letter went UP. Krampus got his letter, but if he is the 'shadow' of St Nickolaus, then Santa/St Nick HAS to be real too? You can't have one without the other! So with that line of thinking, they were saved, but not by Krampus. Nor does Krampus have them as a trophy, but are being watched. What would have been even more interesting for a tag on ending would be after the jump scare KEEP pulling back, through a wall, and find a tall SKINNY sad St Nickolaus in a similar (and cleaner) robe holding his huge olive branch cane, hunched over watching the 'good' elves making toys that look like the ones Krampus uses
Though I have a stupid thing to admit. I swear I remember waking up late on Christmas Eve night like a 6 or 7 year old. I walked down the stairs, the den door was open, and I saw Santa Claus bending over, red butt in the air, putting presents around the tree and I ran back up the stairs as silently as possible. The way the lighting was, was very reminiscent of the lightning when Max wakes up too Of course what I could have SWORN was 'real' must have only been a dream (yet a very vivid silly dream of a child), but the lightning was very similar.
3rd generation American from a long line of Gottscheers it was Drandul, dude!

In a way, Santa is just letting Krampus take his route, because people were being so naughty on Christmas in that area.

You know that in Germany, Krampus is Saint Nicholas's(Santa) partner around Christmas. Saint Nicholas rewards the good, while Krampus punishes the bad.
So why should you be afraid to say Santa doesn't exist? If anything, it makes Krampus seem less real by extension. Still, don't go around spoiling it for them. Let them believe in Santa if they want.

my siblings can handle worse, we watch rated R movies on a daily basis, but Id like to keep the traditon from being spoiled

You are afraid of telling them that Santa os not real but you are OK with showing them this movie that can possibly scar them for life?

Oh my gosh, I remember that as the harshest and most valid reason for a movie character to hate Christmas EVER. I'll never forget it.
There is no objective reality and that's
Sucker Punch

That part of Gremlins disturbed me as a child. Thank God we didn't have a chimney, and you'll have to thank my mother for thinking it was a cutsie wootsie family movie for showing it to a five year old.

You've GOT to be kidding me well then do NOT show them Gremlins!!! There's a whole part where the girl talks about the death of her father inside the family chimney!
3rd generation American from a long line of Gottscheers it was Drandul, dude!

No, it just points out that there's another force on Christmas that isn't as jolly or nice as Santa if you're bad. Something worst than getting coal in your stocking from Santa.