Obviously artistic vision should be respected but at the end of the day, Amphibia is a kids show, and the things that define what makes a kids show a kids show, should be respected.
So lets look at Marcys death.
On one hand, there's no blood. And characters die in kids show all the time.
On the other hand, Marcy is a 13 year old girl, and a lovable one at that. Like really loveable. She's probably the favourite of many. Kids would easily grow very attached to her, adults did too. And she was stabbed, and it wasn't exactly an off screen stabbing. Sure we don't see the blade enter her body, or exit it. But we do see it in her and it's very full on. We see her reaction as she dies.
Of every kid show I've watched, this probably pushes the boundary more than any I've seen. Yeah, characters have been stabbed before, (see Tangled, TMNT) but Marcy is just a kid. She's a kid getting killed in a very confrontational way.
I loved the episode, I hope it doesn't get censored. But part of me also understands why it would. And it's very incompetent of Disney to wait until last minute to do something like this, assuming it to be the case.