And what about asking for neither?
And yes, morality by neccessity involves other people. There is pretty much nothing that a Paladin could do while alone in a desert that could qualify on a moral spectrum without involving another person. And, additionally, what is the difference between "excuses" and "reasons"? Just what the DM judges in the moment?
This ties directly into the point. Why are setting things up for DMs to become judges of morality? Sure, somethings are blatantly obvious, but morality is a quagmire on the best of days, and it seems poorly thought out to try and have someone judge the morality of their friends in a game, especially when you start involving those grey areas and matters of opinion.
And some poisons are completely peaceful ways to die, especially compared to the terror of a man in armor running you through with a length of sharpened steel, perforating your organs and feeling your life blood drain to the ground.
Additionally... some people might not deserve the chance to argue their case or surrender. A king with the power to kill everyone in a room, who has tortured and murdered thousands of people to fuel a demonic portal in hopes of ruling the world doesn't seem like the type of person to confront head on and demand their surrender. They aren't going to say or do anything to change the point that they need to be stopped.
I also fully agree with
@Vaalingrade . The reason poison is considered evil and a "cowards weapon" is because it was the most effective tool of the weak against the strong. Yes, killing someone is wrong, but we give special connotations to those who do so in ways that are subtle or harder to "defend against" without considering that a knight in full armor running down commoners in normal clothes is just as untouchable and unstoppable as the courtier with the poisoned wine. The difference is, who has the money and the power.