Trap only "evil" souls. Be chaotic good.
Soul trap is not legally frowned on, but is potentially immoral. That's lawful evil, not chaotic good.
Indeed, enslaving people (if legal) is the exact opposite of chaotic good.
I've decided to not use black soul gems of any kind, but am OK with white souls. I worship Hirscene and would expect to be hunting them after I die anyhow, so "using every part of the kill" is just good sense.
Serving Hirscene is neither good nor evil, its just an acknowledgement of the power of violence and death.
Guess that' puts my character pretty much in the "true neutral" camp. Which makes sense, its the alignment given to predatory animals in D&D, and is also a good alignment for a mercenary warrior. (I'd started with a tendency towards lawful neutral, but I think I'll be splitting with the blades soon, and I'm unhappy with how Tulious handled Ulfric.)