Although many religions do teach "harm none" Sometimes it is very reasonable, as you may be eliminating something/someone who could harm the world as a whole instead of just to you.
In my opinion, as someone who believes in parts of every religion, and is polytheistic, "harm none" can be a weakness as not everyone follows it. If the enemy you are trying to eliminate has done personal harm to you or your family, in an attempt to harm or kill anyone the rule of "harm none" has already been broken, and therefore as an attempt to protect yourself eliminating this thing/person will not be breaking that rule.
If you want to go further into depth, "None" is directed nouns, such as a person. If you are trying to eliminate a spirit and/or the undead... ect. you won't be breaking that rule either.
I find myself in many battles weekly and sometimes daily, and it is required of me to sometimes kill my enemies, although it is disgusting to take anothers life, as it is not my choice of their fate, sometimes it is necessary. As long as killing does not become an obsession, using a death spell is fine, although I do not recommend it for the average caster, as it takes much energy and power and can often turn on you.