Ensara

"I wasn't sure if you could safely kill something once you've made it part of your soul. Because once it's there, it's part of you, isn't it? Or is that not how souls work in whatever world you picked that trick up from? I've not done a lot to directly interact with my soul, but if I removed the thing I added I'd die." The gold spike stitching his healing ability to his soul goes through his heart. If he'd put it somewhere else, he could potentially remove it without dying and lose the healing ability in the process, but hemalurgy is a messy and complex art. Edward doesn't know enough about it to know what other bind points would work, and didn't fancy the process of experimentation. It would also have attracted very unwanted attention, which Edward at the time would not have been able to deal with.