The Professor: Missing Assistant
Daniel can be found in a room just after the first firefight you encounter when leaving Mordin's clinic. A few batarians are threatening to kill him, believing he's trying to spread the plague. You can try to talk them down or encourage a fight but the result will be the same, Daniel's brains will be little more than an insignificant splat on the wall. The only way he comes out of this alive is with the use of your Charm or Intimidation skills. Use one of those and the batarians will let him go. You can now keep your deal and let them walk, or murder them all. The choice is yours as Daniel will still live and Mordin won't too much care how you handled the situation (though he likely would have killed them all).