Re: I've been wondering...
A mentally-ill woman who already killed and injured several of their friends and loved ones and who, it should be noted, has the power to re-make reality. Charles Xavier, the most powerful telepath on the planet, says he can't keep her incapacitated indefinitely, that sooner or later, they'll be right back to where they started, only with Wanda at full capacity.
Not one of those German soldiers possessed the same power or threat that the Scarlet Witch does at this moment, and Cap still killed them. Yes, there is a difference between war and killing Wanda here, but the threat posed is on the same level; Marvelverse Nazi Germany had a whole bunch of hoodoo abilities and the intention to reshape the world to how they saw it. They had to be stopped before they unleashed (even more) death and suffering.
That sounds very similar to the problem they're facing here, yes?
Cap tried to reason with Wanda before; she tried to kill him and went berserk in an attempt to slaughter the rest of the Avengers. What do you do, then? Do you let this incredibly powerful, incredibly dangerous person/state free reign until the next time they tweak and start killing people? Or do you do what is necessary to keep them from that, no matter what it costs? Even if that's your own principles?
Clinging to your principles at the expense of those around you isn't a particularly heroic act. It's the decision of a zealot or fanatic, to refuse to compromise themselves "even in the face of Armageddon", to borrow a phrase. Yes, Wanda is mentally ill. Yes, this should be a difficult decision. Yes, it should be something debated and argued over, even heatedly. Is killing her the 'right' thing to do? Maybe, maybe not. But does it matter, when she already has killed - and has the potential to kill thousands, maybe millions of people - you should at least be willing to entertain the possibility that, yes, maybe killing her is necessary, if not right.
To blithely state "there's always another way" like Cap has done here is naive at best and willfully ignorant at worst. He can believe that killing Wanda is wrong, he can argue against it as much as he wants; there's absolutely nothing wrong with that. But to completely shut down an option, simply because it's personnally reprehensible is irresponsible. Especially coming from a man who has killed many times over, specifically to prevent the kind of thing Wanda's power and insanity represents: the re-shaping of the world to fulfill a madman's vision.