Sacrifices give a quick, very large, amount of power immediately. 'Pure' souls (virgins, children, white knights, etc) provide a lot more. However in the very long term, you get more in worship.

After death, currently the souls would just go into one of the neutral spirit realms since you don't have somewhere to guide them to. You could trap them in like a gem or something which would provide what you want (I think) but they wouldn't be happy about it.