Just to clear up this matter once and for all; No, you don't have to kill anyone in Deus Ex. The game forces you into positions where you have to fight and kill people on the odd occasion, but they can all be avoided.
Lebedev; Anna tells you to kill him, you can choose not to, Anna scolds you for being a pussy and kills him herself.
Anna; Encountered in the Hells Kitchen subway, which you can just sneak out of through a vent or run past altogether. The moment yoou run into Gunther you are captured.
Gunther; Encountered in Paris cathedral. Can be avoided/ran away from completely.
Maggie Chow; Tranquilize in the UC.
Walter; Encountered in the sub base and Area 51. Can be avoided completely.
Bob Page; Never killed on-screen, presumably dies during one ending and maybe after the other 2, but it is never seen or confirmed, so doesn't count.
Everyone else can be avoided or tranquilized. Though if you attempt to doa no-kills run, Paul will always die when you leave him behind, the only way to save him is with some SERIOUS gunplay.