There are several bugs affecting the Murderous Flames Paragon Feat in the Scourge Warlock Fury tree.
First of all, it has a bugged interaction with the Killing Curse Feat in the same tree. When attacking a Cursed target with Killing Flames, the Killing Curse damage applies first, and then the Murderous Flames damage is based on the Killing Curse damage rather than the true/main Killing Flames damage.
Next, it has a bugged interaction with Armor Penetration - Armor Penetration does not always apply.
In addition, Murderous Flames sometimes does not apply its damage when the target of Killing Flames is killed by the main damage of Killing Flames.
Finally, Murderous Flames is either doing too much or too little damage. In every test, Murderous Flames deals a total of around 50.7% of the damage of Killing Flames (this is distributed to multiple targets) rather than 30%. (Furthermore, 50.7/150.7 is around 33.6%, not 30%.)
I have several screenshots that demonstrate some of these bugs.
Killing Curse applies first, Murderous Flames deals the wrong damage to two targets, does not respect Armor PenetrationKilling Curse applies first, Murderous Flames deals the wrong damage to one target, does not respect Armor PenetrationAttacking a non-Cursed target does not proc Killing Curse, so Murderous Flames deals its extra damage based on the correct hit, but still does not respect Armor Penetration