Saren Arterius. Let it be known that he was a Complete Monster beforegetting indoctrinated by a reaper, that he seemed to enjoy going Knight Templar on targets and even innocent civilians. His mass-murder on Eden Prime was only the most extreme act of calumny on his part. Unless you count him deliberately trying to bring back the Reapers...
Dr. Saleon, a salarian geneticist who sold black market body parts and organs. How did he get these organs? He hired people off the street and grew the organs inside of them, making them living test tubes. If the organ didn't work out, he would leave it inside of the person.
Oh, and if a Paragon Shepard convinces Garrus to arrest him instead of just killing him, he responds to this generous act of mercy by trying to kill them with a dingy rifle. So not only is he a monster, he's Too Dumb to Live to boot.
Immoral scientists always seem to be these in the Mass Effect universe.
There's also Nassana Dantius, the corrupt diplomat from the first game. In the sequel, she's left politics and is now a Properly ParanoidCorrupt Corporate Executive who orders her Mecha-Mooks to gun down her own employees out of paranoia. Even Paragon Shepard is glad to see Thane kill her.
She ordered one of her soldiers to hunt Shepard's team down, but he was Genre Savvy enough to refuse. Once she convinced him otherwise, she randomly killed him and threw into a hole that led to the bottom floor.
One of the workers you end up saving has this to say:
"Tell your assassin to aim for her head, because she doesn't have a heart!"
It takes a special kind of evil to disgust people despite being dead, but the Teltin scientists manage to pull it off. While you have to have a certain degree of moral ambiguity to work for Cerberus, but these guys managed to make The Illusive Manorder them killed for being so evil. What did they do? Kidnapped biotic children from across the galaxy, including manipulating mothers into thinking their children died from eezo posioning and then getting them to sign the "body" over to them while they're traumatised. Once taken back to their facility, they experiment on the kids, making them fight each other and using drugs to condition them to enjoy violence, mentally torturing their main subject by putting her in a room with a one-way mirror so she could see children outside her room, but who could not see or hear her cries for help. And when it all goes to hell, what is their priority? To try and join the Alliance's program to repeat the experments all over again (which they would have done had The Illusive Man not decided that their experiments were too twisted, even for him). It's easy to see why Jack is the way she is from a childhood like that, and the other survivor is even worse off mentally.
