First of all, we have to remember that Mann was a coward. Upon arriving on his planet, he realized it was uninhabitable but didn't want to die alone, and as such sent out a beacon to say he had found a Goldilocks habitat, so a rescue mission would be launched to find him.
Cooper, believing that they had found a suitable habitat for colonization (based on Mann's lies) was determined to take the shuttle through Gargantua and back to Earth. For all intents and purposes, at this point they believed the mission goals to have succeeded.
Cooper was going to leave them on the planet and return home.
However, Mann was lying about the planets habitability: it was, in-fact, uninhabitable. Mann was acting in self-preservation, and his options were either to somehow gain Coopers trust, assume the pair of them could overrule or overpower the rest of the crew, and return to Earth having failed the mission: or, to kill Cooper when he is vulvernable and unsuspecting. His plan from here is largely undisclosed, and he was possibly only acting spur of the moment, killing Cooper when the opportunity arose to stop him abandoning the remaining crew on an inhospitable planet.
Because Mann lies, it is unclear whether he intends to return to Earth alone in the shuttle, or (as he broadcasts to the crew) to "complete the mission" and find the last hospitable planet to start a colony. His cowardice would indicate the former, however.
Basically, cooper was going to leave the crew abandoned, and Mann saw the opportunity to eliminate him. I don't think this was premeditated, but when Coop asked Mann to escort Coop to the monitor stations, the opportunity was too good to pass up.