Earlier this week the Israeli High Court of Justice held that the practice of "targeted killing" of terrorists by the Israeli military is not prohibited by customary international law. As Julian Ku explains on Opinio Juris, the opinion held that international law "does not prohibit all targeted killings of Palestinian terrorists, but that it might prohibit some such killings," and it asserts the right of Israeli courts to review the legality of such actions in the future.
It appears that the Israeli court adopted a position generally in line with that advocated by my colleague Amos Guiora, a veteran of the Israeli Defense Forces and Director of the Case Institute for Global Security Law & Policy. Amos has contended that targeted killings are permissible in certain lmited circumstances as a form of "active self-defense." Specifically, preventative strikes against known terrorists who are preparing for additional attacks, is permissible, provided that such actions are proportional, do not needlessly endanger civilians, and are the result of a process designed to limit the likelihood of mistaken identification of terrorists. This is a controversial view in international law, not least because it rejects the "law enforcement" paradigm for counter-terrorism measures.
It is important to reiterate, however, that the opinion did not give the Israeli military a blank check. Rather, it held that targeted killings as such are not, in and of themselves, contrary to international law, while leaving open the possibility that specific attacks, particularly those that are disproportionate, would violate international law. The LA Times has more on the opinion here.