I feel sorry for Revenant

Revenant is portrayed as a sadistic, evil and bloodthirsty literal killing machine but for me his lore is by far the saddest.

"Revenant used to need a reason to kill. But he's not that man anymore."
This quote from Revenant's lore page at playapex.com shows that even as a simulacrum, Revenant had some sort of honour code despite his profession. He wouldn't take lives unless he had to (as seen in the Assimilation Launch Trailer where it is heavily inferred that he didn't want to kill Loba's mom but she got in the way by shooting him). When he found out he was a simulacrum, he became psychotic and felt the need to take his century-lasting pain and agony out on people. 

"You should thank me. You can rest now... I never will."

This kill quip of Revenant's shows that he feels self-pity and just wants to finally die and end his eternal agony. You can also hear a bit of sorrow when he says the words 'I never will.'

"The sheer terror that comes with dying... I remember it well."

Revenant essentially reveals that whenever he died as a simulacrum, he felt the agony and fear of death and since he cannot die, he had to feel that repeatedly. When he learned that he was a simulacrum, it seems that his memories of dying over and over again came back to haunt him and caused him to become a psychopathic murderer.

What's even sadder is that he finally thought his one wish could come true and Loba backstabbed him and told him he'll never see his source code again. Loba is so blinded by her lust for vengeance that she doesn't even understand she is in the wrong, not Revenant. She doesn't understand how Revenant works nor the pain he feels.