"Killing a captive who poses no immediate threat is a crime under military law as well as all other law," he told the BBC World Service.
"As a matter of international law, one country is not free to enter another country apparently without the authorisation of that country, and intervene, whether to kidnap or kill a national of a third state," Mr Sands said.
The UN special rapporteur on extrajudicial, summary or arbitrary executions, Christof Heyns, and the special rapporteur on the promotion and protection of human rights and fundamental freedoms while countering terrorism, Martin Scheinin, have raised a similar concern.
"However, the norm should be that terrorists be dealt with as criminals, through legal processes of arrest, trial and judicially decided punishment," they added.