The Ironic Role Reversal Between Sigma and Zero
In Megaman X4, I do find Zero's storyline pretty weird when his backstory was revealed. In the beginning, we finally see that Zero was created by Dr. Wily to defeat Megaman and much possibly, to succeed him. Zero was sealed away but was probably programmed to fully activate when Dr. Wily dies, to continue his "father's work" which he was that "Red Maverick" referred to by Sigma. As a Maverick, Zero was petty and cruel to which he even nearly killed Sigma, prior to Sigma becoming the big bad.
The irony is also this- Sigma was once the honorable leader of the Maverick Hunters but ends up becoming the Megaman X series' antagonist effectively replacing the late Dr. Wily. While Zero was supposed to succeed Dr. Wily, he joins the Maverick Hunters. Since Megaman X has no memories of his past or past existence as the original Megaman, it wasn't hard for Zero and X to work together. I mean Megaman X was Dr. Light's creation, Zero was Dr. Wily's successor (since Bass was disloyal to Dr. Wily). When Sigma punched Zero's gem in one of those "seizure moments" which was probably caused by a programming flaw that was later fixed, the Evil Energy entered into Sigma. For Sigma, he apparently rebelled at his own free will and having control of the Evil Energy that was later known as the Maverick Virus. So it's possible that EVERY LAST knowledge Zero had in his head went to Sigma in the process. So Sigma instead ends up succeeding Dr. Wily instead of Zero, Dr. Wily's heir designate.