We have always taken this to mean that any harmful act requires malicious aforethought, and that without such thought the act would not occur, hence the real sin is in the lead up, not the actual act.
Even harmful acts as the result of accidents require malicious aforethought to this degree: Someone has either ignored or refused to acknowledge potential harm in their actions because doing so would interfere with their desires.
So there's plenty to anchor Christ's teaching in the here and now of this universe.
But what about multiverses?
One of the startling / frightening implications of quantum theory is that every choice, every action, every act even on the random sub-atomic particle level splits off a whole new alternate universe.
Somewhere a infinite number of Constant Readers are looking at this and thinking, "Buzzy-boy's finally gone fug-buck".
And somewhere else another infinite number of Constant Readers are looking at this and thinking, "Yeah, I can see how that would work..."
And another infinite number of Constant Readers are on an entirely different website...
And another infinite number of Constant Readers couldn't access this page even if they wanted 'cuz in their multiverse it doesn't exist.
And another infinite number of Constant Readers have their computers turned off and are outside playing...
And another infinite number of Constant Readers never even existed...
So maybe what Christ was getting at was this: In this universe, in time and space as we humans understand it, the desire to do evil is all that is needed for an evil act to exist.
But in addition to merely harboring an evil desire in this world, we create an alternate multiverse where such a desire was actually acted upon.
Puts a lot more onus on keeping our hearts and minds on things of the spirit, huh?