Strongest argument against death penalty is the Thai justice system

Capital punishment has not deterred crime, yet we keep convincing ourselves otherwise. Remember Thaksin’s “war on drugs” of over a decade ago, with a body count of 2,500? Yet, 64 per cent of Klong Prem’s 6,267 prisoners (as of July 2016) were there for drug-related crimes. That’s not a successful “war”.
Secondly, Dartmouth College researched the question: “Does capital punishment ... provide a better deterrent to murder than long imprisonment?” It found that “ an overwhelming majority among America’s leading criminologists ... believe that capital punishment does not contribute to lower rates of homicide. … The consensus is international in scope.” (.https://www.dartmouth.edu/~chance/teaching_aids/books_articles/JLpaper.pdf).
Thirdly, in the June 19 episode of TV news show “Kom Chad Luek” on capital punishment, even the proponent of capital punishment admitted that execution does not reduce crime.  
The purpose of court sentences is (a) to deter and (b) to punish. Executions haven’t deterred, but should we retain them for the punishment effect? 
But no justice system is 100-per-cent just. In Thailand, for example, two Myanmar men are now on death row for the grisly Koh Tao rape/murder of a British couple. This despite the fact that their conviction was based mainly on DNA analysis by a lab not accredited to do such analysis, that there were gaps in the chain of custody for the DNA samples, and that the very complex DNA analysis was declared complete in a physically impossible time span. Any one of these deficiencies should have rendered the analysis inadmissible in court, yet the verdict was upheld on appeal. Is our justice system 100-per-cent just?
Instead of capital punishment, I suggest that we substitute life imprisonment without possibility of parole or pardon, which would fully protect society yet allow for a retrial if evidence of innocence was eventually uncovered.
Burin Kantabutra