Okay, while it's true that you can take a bit of someone's energy and use it in spell work, it's not something you can constantly be doing if you aren't near the person (and it would probably start being draining for you if you did, therefore defeating the purpose).
No, you cannot constantly be taking ten percent of someone's energy and putting it in a sigil.
Also, though I am not an expert in sigils, I am fairly certain that charging them, "powering" them, or infusing them with any sort of energy or intent is redundant. I have not heard of anyone doing that. I may be wrong about that part, though, so take it with a grain of salt, and anyone more experienced in sigils is free to correct me.
However, whether it's necessary or not, your energy is enough. I use my energy to charge crystals and it works perfectly well.
I'm unsure of how a soul would protect you from a karmic law. If you believe in such things, nothing is going to protect you from it forever. If you don't, you don't need to worry about it anyway. Also, most people say that souls cannot be purchased, though that is debated.