That can never be a Democracy. Or any such act cannot be denoted as a democratic act. Killing anyone (despite being at fault) is not allowed to be done by any individuals. It is illegal to law into hands. Legal processes can be adopted to assure the enforcement of justice. The leaders are elected by the common citizens, so they must vote for some better option. But killing someone still cannot be allowed at any case.
First thing that every gov will assure is to be safe from plebs. So it is impossible to adopt legal processes, simply because it doesn't exists. So of course, nobody will have a real right to kill a leader, it is up to you to do it even if you don't have those
rights. But count on that - violent elimination of gov will always be iligal.