[4.91] You will find others who desire that they should be safe from you and secure from their own people; as often as they are sent back to the mischief they get thrown into it headlong; therefore if they do not withdraw from you, and (do not) offer you peace and restrain their hands, then seize them and kill them wherever you find them; and against these We have given. you a clear authority
EXPLANATION Verse 4:89 simply refers to hypocrites who were treacherous and were helping the enemy. And the punishment for treason, even according to the US laws, is death. Verse 4:90, often ignored by critics of Islam, clarifies the issue:
[4.90] Except those who reach a people between whom and you there is an alliance, or who come to you, their hearts shrinking from fighting you or fighting their own people; and if Allah had pleased, He would have given them power over you, so that they should have certainly fought you; therefore if they withdraw from you and do not fight you and offer you peace, then Allah has not given you a way against them.