The use of soul gems is a horrendously disproportionate and unjust response to the situation.
If someone comes after you on the street with a gun, fully intending to kill you, you're well within your rights to pull out your own gun and kill them first. Taking the life of an aggressor in order to preserve your own is proportionate and morally justifiable.
You are not within your rights to subdue them, throw them in the trunk of your car, take them out to the boonies and lock them in a shed, forcing them to donate blood over and over again until they die.
That is a disproportionate response to the situation and turns you from a victim operating within his rights to a criminal violating the rights of another person.
Once your aggressor was subdued, they were no longer a threat to you and the conflict ended at that moment in time.
You then went and started another conflict (in which you were the aggressor) by kidnapping them, unlawfully imprisoning them, violating their right to bodily integrity by forcibly extracting blood from them, and finally killing them from the physical strain of your previous crimes against them.
It doesn't matter if you used the donated blood to save the life of Steven Hawking, or some researcher who was on the brink of curing cancer, or the guy who had the codes to shut down an impending nuclear attack.
You're still a kidnapper, a torturer, and a murderer, and the jury will rightfully see that you're buried under the jail.