So I've been at work with the giant death machine and I've been trying to do it on my own without looking up the "right way" to do it, why? Because I'm just too damn stubborn to do it! I did take a look at the way the game handles spawns which is critical to know that you need to be 24 blocks away in order for a mob to spawn. Good to know in a cave too.
The tower itself now looks like a giant mushroom, and technically it does work, but the death mechanism needs some work, I guess I'll go get some lava and do it like that. I was really hoping to keep with the cactus like in the first death machine, but it just doesn't seem to be doing the trick.
Oh well I'll keep tweaking it until it gives me what I want, dead mobs. Maybe I just need to keep going bigger?