Yes, using poison is inherently evil (if poison is Evil, as with traditional D&D worlds). Anything other than a straight-up, stand-there-and-swing-at-each-other fight is dishonorable, and putting something on your blade that isn't just your own fighting ability is the height of dishonor. The world of D&D is not a subtle or nuanced one. There are omnipotent beings that watch your every movement and hold judgment over you. If poison is Evil, then no technicality will allow you to bypass Cosmic Law.
If you wanted to incapacitate a guard, you could do that by simply striking to KO rather than kill.
Well then I'll just have to quietly and nonlethally incapacitate guards nonlethally in an evil way then.
I love that killing people is not inherently evil, but being pragmatic is.
