In The Adventures of Dr. McNinja, Dr. McNinja is having trouble battling the evil Frans Rayner, because all the McNinja clones (long story) are triggering Conservation of Ninjutsu. So the Genre Savvy doctor does the only thing he can — team up with Frans and start taking the clones out himself. Yes, Face-Heel Turn as a fighting tactic. The clones were going to die either way, and Doc would have died with them if he hadn't changed sides. Besides, he went back to Face when there were no more clones, because Conservation of Ninjutsu was no longer in effect (and Rainer would have killed him had he stood quiet).
Eridan from does this after deciding that the fight against Jack Noir is completely hopeless and as the "Prince of Hope" he believes he is the most qualified to determine if all hope is lost. He figures the only chance for survival is to pledge his loyalty to Jack. He then proceeds to K.O. Sollux, kill Feferi, and finally kill Kanaya (while destroying the key to restoring the Troll race) when they try to stop him before he escapes.
And then, not long after, Gamzee runs out of sopor slime pies and becomes sane, which causes him to remember his destiny as a murderous Subjugglator, and is later seen working for Caliborn/Lord English.
Synn from the The Law of Purple performed one of these before the start of the comic. It didn't take very well, tho, since it took little more than a few words from his old best friend before Heel Face Turning back.