お前も私も一人にせず、且つお前に 少しの 罰を 与える 方法を考えてみた。
In this phrase I've some doubt about "一人にせず". Using google, I can translate it as "not alone". But I read "せず" is a negative form of "する” and indicate that one action took place without or in the absence of another action. But... It seems senseless.