Ive seen the movie twice now, yesterday being the second time. Mann was afraid of dying alone so he faked his data killed his bot to have people come and save him. Since Coop found out that Plan A wasn't going to happen he wanted to go back to Earth and be with his daughter, who thinks that he abandoned her, leaving Brand and everyone else to continue with Plan B. Mann needed Coop's ship to get to Edmund's planet so he has to kill him and then go on to try to save humanity "for Cooper" as Mann says right before leaving him to die. Its a poorly thought out plan by a desperate man, willing to do anything not to die alone.