Kidnapping innocent person = evil, yup. Accidentally killing her is icing on the cake.
Edit: As has been said, this sounds a lot like a Coen brothers plot. As GM I'd say this evil act would be enough to switch Good-aligned PCs to Neutral, but not Evil, straight off. Certainly it was a somewhat Chaotic act also.
Also, I don't agree it was a bad GM call that the crit killed her - striking to subdue is still striking for damage, a crit is extra damage. A fumble would be missing her & breaking or dropping your weapon, or hitting another party member, etc.
Edit 2: Having said this doesn't make the PCs Evil per se doesn't mean that as a citizen of the town I wouldn't happily sentence them all to death - they're clearly a major Danger to Society! They're just too stupid to be (necessarily) Evil.
Re 'adventurers routinely commit murder' - it all depends on who you kill, and why. If you take out the goblin raiders who've been attacking local farmsteads, the typical D&D scenario, that's not evil. If you go slaughter the goblins who've never done any harm to anyone, that depends on the nature of goblins in the campaign - in my campaign it might well be evil, since goblins are just as much sentient beings as humans or elves, in some others goblins are inherently evil and killing them is always A-ok.