During the ongoing wave of deadly terrorist attacks in Israel, there reportedly have been a few instances in which bystanders attacked the perpetrators after the latter had been rendered incapable of causing further harm. Shlomo Brody presents a halakhic perspective on such behavior:
The [talmudic] sages contend that the verse “You shall not stand idly by the blood of your neighbor” (Leviticus 19:16) not only demands saving a friend from drowning or from a hungry lion, but further dictates stopping an assailant from committing murder. This mandate was extended to both threatened victims and bystanders alike. . . .
Jewish scholars debated what guidelines might be issued for those trying to stop the assailant. . . . [I]t is not always easy to know when a threat has been fully neutralized, especially when a wounded terrorist continues to struggle. Nonetheless, [there is a general consensus that] once the assailant is clearly incapacitated, one is not allowed to kill him, whether out of [a desire for] vengeance or [a sense] of vigilante justice.
Read more on Jerusalem Post: http://www.jpost.com/Not-Just-News/Ask-the-Rabbi-May-one-kill-a-subdued-terrorist-436204