Rationalizing cold-blooded murder is something for sick minds and for the greatest of writers.   Shakespeare was able to imagine how a flawed personality might rationalize the assassination of a friend for something the intended victim had not yet done, but might do. Amoral? Sick? Reprehensible? Yes, all of these. And also the topic for the first great soliloquy Shakespeare ever wrote:

Brutus
It must be by his death, and, for my part,
I know no personal cause to spurn at him,
But for the general. He would be crown’d:
How that might change his nature, there’s the question.
It is the bright day that brings forth the adder
And that craves wary walking. Crown him that,
And then, I grant, we put a sting in him
That at his will he may do danger with.
The abuse of greatness is when it disjoins
Remorse from power, and, to speak truth of Caesar,
I have not known when his affections sway’d
More than his reason. But ’tis a common proof
That lowliness is young ambition’s ladder,
Whereto the climber-upward turns his face;
But when he once attains the upmost round,
He then unto the ladder turns his back,
Looks in the clouds, scorning the base degrees
By which he did ascend. So Caesar may;
Then, lest he may, prevent. And, since the quarrel
Will bear no color for the thing he is,
Fashion it thus: that what he is, augmented,
Would run to these and these extremities;
And therefore think him as a serpent’s egg
Which hatch’d would as his kind grow mischievous,
And kill him in the shell.

—Julius Caesar, Act 2, Scene 1

Before we get to the why this soliloquy is the first of its kind, note the imagery: There are two metaphors here. Caesar as a serpent, and Caesar as a ladder-climber. The ladder-climber has become part of our common vernacular today–“climb the corporate ladder.” The other metaphor, though–Caesar as the adder–is the creepier of the two, and Shakespeare has Brutus return to that image in the final, decisive line: “And kill him in the shell.”

That metaphor allows Brutus to develop the rationalization to murder Caesar not for anything he has done, but for what he might do. That is, kill the snake in its shell. As cold-blooded as you can get. Yet–it’s okay, he’s rationalized it.

Finally, as mentioned above, this was Shakespeare’s first great soliloquy. The idea of being able to convey on-stage a character’s innermost thoughts was something never before done in theatre until Shakespeare tried it out in Julius Caesar–all the way back in 1599. This speech is the first one. 

It worked so well for Shakespeare–it so illuminated the inner workings of his characters–that he then refined the practice. To this very day, we remember even greater soliloquys that followed in his later plays: ‘To be or not to be’ from Hamlet, or ‘Tomorrow, and tomorrow, and tomorrow’ from Macbeth, just to name two. This technique changed the nature of art forever. Need examples? It’s used today in popular culture: think of ‘The Office’ or ‘Modern Family’, where characters are given the opportunity to speak directly to the audience.

The image comes from the 1953 film version of Julius Caesar, with James Mason as Brutus (right) and John Gielgud as Cassius (left). Note their post-assassination bloodstained tunics, in glorious black and white.