WARNING: The following article contains spoilers for Fantastic Four #33 by Dan Slott, R.B. Silva, Luca Maresca, Jesus Aburtov and VC's Joe Caramagna, on sale now.
If there was anything more surprising than Doctor Doom getting married, it was his follow-up plan to forgive his enemies for any and all past transgressions. The whole idea behind his wedding wasn't love, but rather to ensure a secure and stable Latveria. Giving it a strong queen would have ensured the country's internal security, but making peace with his numerous enemies would have protected the country from outside threats as well. It could have been a new era for Latveria and Doom himself. But all of that was foiled by the weakest member of the Fantastic Four: the Human Torch.
Johnny Storm was having relationship issues and found himself having an intimate moment with Victorious, Doom's personal champion soon-to-be fiancé. Doom proposed seconds after Johnny and Victorious were getting out of bed, but they both knew that her accepting his proposal was a problem. Victorious had technically cheated on Doom with one of his worst enemies. To make matters even worse, it was an enemy he doesn't take particularly seriously.
But Victorious is an honorable sort and she told Doom the truth of what happened right there on the altar. Naturally, Doom didn't take it well. In his fury, Doom attacked all of his guests and ordered that the Fantastic Four be executed for their crimes against his house and honor, thus ending any chance for peace between Doom and Marvel's First Family. What makes it even worse is that, in terms of threats, Doom probably views Johnny as the least imposing of the Fantastic Four.
Johnny is powerful, make no mistake. But his powers are more wild and uncontrolled, something Doom abhors. Latveria's king prefers powers that can be used precisely to devastating effect. It's why even though he's a master of magic, Doom prefers science because it's more controlled. Johnny is just fire, once loosed there is very little control over it. He doesn't have Reed's intellect or Susan's raw power or even Ben Grimm's strength. He's just the guy who sets things on fire. And in this case, he's set Doom's plans ablaze.
So killing the Fantastic Four at this point is more about saving face. He was just humiliated on an international scale by the least intimidating member of the Fantastic Four. Ironically, this means that Doom was embarrassed by both Storm siblings on a global scale. But this time, it isn't just a matter of his personal image that is at risk. Latveria's future queen just admitted to having an intimate moment with one of the country's sworn enemies. Someone they were about to forgive, but nevertheless this happened before forgiveness was even on the table. To ensure that his country can never doubt their future queen again (if he still wishes to go through with the marriage) Doom will need to eliminate any potential for this embarrassment to happen again.