I want to replace the walking animation and the death animation.
When a character spawn, they come with an Animate script that creates and loads the preset animations. All you have to do it destroy that script when a character spawns and replace it with your edited version.
And for death, you replace shoulder and hip Motor6Ds with Glue welds for a rag doll.
Otherwise, remake all the joints when a character dies and run your death animation when the humanoid dies.