When the effect resolves, even if the number of cards in your hand/field has become more than the opponent's, the "destroy all other cards on the field." effect is still applied. ("your opponent takes no further damage this turn" is also applied.)
The effect of "Evilswarm Exciton Knight" cannot be activated more than once in the same chain. Also, if not on the same chain, after resolving the effect of "Evilswarm Exciton Knight", you can activate its effect again that turn. (However, you can only activate it a second time if your opponent's cards in the hand and on the field still outnumber yours.)
↑Konami OCG Card Database: If the effect "Evilswarm Exciton Knight" is negated by "Skill Drain" or the effect of "Light and Darkness Dragon", is the effect that reduces all damage the opponent takes to 0 still applied?