Why is the UK legal system relevant here? Well, it's really not, but I am just comparing it to a legal system that I am familiar with, which is a bit of a joke as well; I am happy to hear how the legal system works where you are from if you are willing to tell. Additionally, the Chinese legal system uses civil law, which is based on the Roman system that is used throughout Europe, I mean if I had a spin of it in the UK, can make it look like a horrible system, even though it works.
As for what's easier, let's look at it this way. For me to kidnap someone, I can just find where they are staying, break into said place and then take them away; if I am super careful, I will do it when there is no one around. For me to create a legal extradition route, then I would need to look into your history, financial dealings etc, find out somewhere that looks dodgy, and try to use that as evidence; sometimes I cannot even use that as evidence given it may not even be committed in China. Remember, Al Capone was sentenced not due to the other crimes he committed (due to lack of evidence) but rather due to his tax evasions, so proving something legally when you haven't done it is very hard. If given that you can just go around "kidnapping and disappearing people" as you have stated, why would you want to bother create the legal route right?
As for what's easier, let's look at it this way. For me to kidnap someone, I can just find where they are staying, break into said place and then take them away; if I am super careful, I will do it when there is no one around. For me to create a legal extradition route, then I would need to look into your history, financial dealings etc, find out somewhere that looks dodgy, and try to use that as evidence; sometimes I cannot even use that as evidence given it may not even be committed in China. Remember, Al Capone was sentenced not due to the other crimes he committed (due to lack of evidence) but rather due to his tax evasions, so proving something legally when you haven't done it is very hard. If given that you can just go around "kidnapping and disappearing people" as you have stated, why would you want to bother create the legal route right?