Voidforged Paladin's effect is fairly straightforward when you put it in play and defeat a Monster. However, what happens if you defeat a Monster, put Voidforged Paladin into play, and defeat a second Monster? Does the effect trigger?
The short answer is, "Yes". Cards that include the text, "The first time...", only have their effect trigger once it's put into play. In other words, the card has no "memory" of actions that have taken place prior to the card being put into play.
So, in the case of Voidforged Paladin, it would not be aware that you previously defeated a Monster in the Center Row. If you've defeated a Monster, played Voidforged Paladin, and then defeat another Monster, the effect will trigger in full.