What it boils down to is whether a use of force in self defense is "reasonable" considering all the circumstances.
"The [defendanats] need not have been in actual danger of great bodily harm, but they were entitled to act on appearances; and if they believed in good faith and on reasonable grounds that they were in actual danger of great bodily harm, although it afterwards might develop that they were mistaken as to the extent of the danger, if they acted as reasonably and ordinarily cautious and prudent men would have done under the circumstances as they appeared to them, they were justified in defending themselves.
STATE v. McCULLUM, 98 Wn.2d 484, 656 P.2d 1064 (1983) (mrsc.org).
You can read the jury instructions here: http://weblinks.westlaw.com/toc/default.aspx?Abbr=WA-WPIC&Action=expandtree&AP=I6e2b6c50a9e711da978f9558dabefcda&ItemKey=I6e2b6c50a9e711da978f9558dabefcda&RP=%2Ftoc%2Fdefault%2Ewl&Service=TOC&RS=WEBL12.07&VR=2.0&SPa=wcrji-1000&fragment#I6e2b6c50a9e711da978f9558dabefcda
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