I cannot fathom how a "lethal injection" could have been botched, unless it was done deliberately by the doctor or technician as a protest against capital punishment. The usual method used in those cases of "assisted suicide" involves a sequence of three lethal drugs injected into an IV line. First, barbiturates, which is a sleeping pill, but a large dose which would kill 99.99% of the population. Next, a paralizing agent like curare which causes a complete and total paralysis of your breathing muscles, which should kill 100% of the population. Third, a huge dose of potassium which makes muscles incapable of functioning. There might be a variation of the individual drugs used but I have never heard of this method not working.
I would not object to the capital punishment of Adolph Hitler or someone just as evil and cruel. In the US, some capital punishment cases go on for decades with dozens of appeals with both the defendant and prosecuter dying of old age. And all the lawyers get millions of dollars from the taxpayers. In my opinion, give these people life without parole. I'm sure many people would actually prefer a quick painless death to the prospect of spending the next 20 plus years in prison.