If you're told that two people have once driven over the limit, that A killed someone while B didn't, and nothing more, what's your level of credence that B is the more skilled drunk driver?

Pretty high, actually. Drunkenness is a red herring here. Let's put it another way: if you're told that A once killed someone accidentally while B didn't, what's your credence that B is better at friggin' not killing people accidentally? You seem to imply the credence should be low. Why on Earth? I say it's pretty high, because A has demonstrated a very low level of said skill.