How did roman generals enforce decimation?

I knew the term before and that it meant to kill 1/10 of an army as a punishment but I just watched a video in which it got explained a little more and now I have questions.

So someone fucked up big time and now I get ordered by some asshole to brutally and cruelly murder 10% of all the people I have known for the last couple of years? Or even worse they are killing me? Why dont we just kill the asshole and find someone who wont mass murder their own soldiers to lead us? If we are going home and someone asks where our general is we can pretend that some assassin or disease got him.