Confusion with item NBT

I need some help. I'm trying to make an item that stores 'souls' as a numeric NBT value on itself, but it seems that the number resets every time I update it.
My script is as follows:

The message sending is only for testing. When the event triggers, it tells me that the item had 0 souls, but now has 1 soul. Upon the event firing a second time, it tells me it had zero, but now has 1. I do not know what is going wrong. It always goes from zero to 1, no matter how many times I slay an entity.

Please help,

Diskman