Currently, summoning the infernal will correctly aggro the infernal onto you after dismissing the pet (thus, making it an enemy). Should the infernal kill you, or otherwise lose aggro of you, the infernal should direct its attention towards any other nearby target, be it friendly players or enemies. Currently, after it has lost aggro of the warlock, the infernal will not lock on to any other target (unless it (the target) attacked the infernal first), after a very short period of time the infernal then despawns leaving no trace behind of said infernal.
In the following bugfix posted just about 20 days ago, it states that this should have been fixed:
To see the original functionality, see this youtube video of a player releasing an infernal in Goldshire, where it does not immediately lose aggro and despawn, but rather engages players in an interesting fight for their lives:
This was recorded in november of 2006, which means still in 1.12.2.
I also did not see anything about this in the known bugs list, so I wasn’t sure if people were aware of it or not.