Happy Tree Friends is devoted to this. Every episode features deaths that are extremely gruesome and photorealistic...and yet it looks like a children's cartoon.
There are loads in Rise of the Mushroom Kingdom. Among the highlights are a Koopa burning alive in lava, Mouser's getting eviscerated, Luigi getting torn in half and......Well.....Any time a Yoshi is onscreen.
In Power Star, any time Mario pulls that sword out of nowhere, everything dies in painful ways. And those are the mooks, things get freaking violent when the main characters start dying.
In Super Mario Bros. Z, Mecha Sonic loves killing his opponents in the most painful and brutal ways possible. Just ask Sonic's friends, the Koopa Bros, and especially the Axem Rangers X.