The controller didn't lose his job or go to prison. He was just put on leave, IIRC. So the killer did the right thing, since the system failed.
It also doesn't matter if his mistake was intentional or not, since it resulted in so many deaths. If the controller had any honor at all, he would have committed suicide. Since he didn't, he needed to be killed.
Not everyone has a sense of responsibility. Just look at the people running our government and financial institutions. Now that this controller's been eliminated, it'll probably make other air-traffic controllers do a better job or find another job if they can't hack it.
That's fine, so long as I can execute drivers who speed, run yellow lights or stop signs, don't yield to pedestrians in crosswalks, and who even think of drinking before driving. With 40,000 fatal car accidents a year in the U.S.A. alone, perhaps if we execute some of the 25,000,000 drivers who are issued citations a year (perhaps 1 in 10 like the Romans - that's only 2.5 million a year), we might reduce the accident rate! It doesn't matter if thier mistake(s) were intentional or not, since they result in so many deaths. /sarcasm
Air traffic controllers are generally highly skilled, responsible, underpaid professionals who do a thankless job cleaning up messes of overloaded and antiquated systems. If you believe extra-judicial executions are okay for them, guess what - you won't have an ATC system at all, because no one in their right mind would bother. In the real world, where serious adults are interested in systematic safety improvements instead of chest-thumping suggestions of violence, nearly all aircraft accidents have been decriminialized (the exceptions are made for 'careless or reckless operations' or willful violations of law), since nearly every professional in the aviation industry doesn't want to cause an accident, and is more likely to be truthful about all the circumstances that led to the accident if they are not in jeopardy. The results of these more candid examinations of accidents (look up ASAP programs if you want to learn more about them) have lead directly to multiple improvements in regulations, procedures, and the overall safetly level in aviation - in other words, they have saved lives.
Your casual suggestion that executing people is a good way to encourage others makes me wonder whether you are even serious.