I was surprised to learn (in the background for the recent protests) that a Hong Kong resident can murder someone in Taiwan and go back to Hong Kong with no consequences (by the way, the murderer is actually imprisoned now in Hong Kong for a few years because he used the victim's money in Hong Kong, but I want to consider a hypothetical case for this question where he took no money).
Since USA has no extradition treaty with Taiwan (though USA has many more extradition treaties than Hong Kong has), could a USA resident do the same thing without penalty? In other words, even with proof sent from Taiwan and a confession, would USA really not be allowed to send the murderer to Taiwan or somehow penalize him? This is hard to believe, so I wonder if there's a special law to deal with this in USA similar to the law being considered in Hong Kong. I have reviewed this similar question for a less serious crime which makes me think the USA state might be important, so assume California.
Also, every extradition treaty I have read is retroactive (meaning crimes before the treaty was signed can cause extradition). Is that usually true? Is the new proposed law in Hong Kong retroactive?