Does an Ebonblade Reaper holding a Quietus Spike kill players?
No, it doesn't work like that. The triggers decide how much damage to do when they resolve, not when they're put onto the stack.

So if your opponent were at 21 (to make things simple) it'd go like this:


Ebonblade Reaper deals 1 damage, putting them at 20. Both triggers go on the stack.
The first of the triggers resolves, making them lose 10 life, putting them at 10.
The second of the triggers resolves, making them lose 5 life, putting them at 5.


You do get to pick what order the triggers go on the stack, but it doesn't really matter, since they're doing the same thing.