I think that the play Julius Caesar brings across many messages. But I think that the most important message it brings across is that you must never make any important decision based on mere assumptions. This was the only reason Brutus failed to succeed in the conspiracy and also led to his downfall.
Almost all his decisions were based on assumptions. He killed Caesar because he thought Caesar might be tyrant,
But ’tis 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 colour for the thing he is,
Fashion it thus: that what he is, augmented
Would run to these and these extremities;
he let Antony live because he assumed that Antony was weak,
For Antony is but a limb of Caesar.
he let Antony speak because he thought there was no harm done if Antony gave speech in Caesar’s funeral,
It shall advantage than do us wrong.
and he marched down to Philippi just because he thought that the enemy would become stronger in Phillipi.
The people ‘twixt Philippi and this ground
Do stand but in a forc’d affection;
For they have grudg’d us contribution.
By them shall make a fuller number up,
Come on fresh’d, new-added, and encourag’d;
From which advantage we shall cut him off
If at Philippi we do face him there,
These people at our back.
All of the actions were based on idealism rather than practical reasons. His assumptions led to his downfall.
However he is not the only one who made important decisions based a hypothesis. Even Portia killed herself because she thought that Brutus was dead. Cassius also killed himself because he supposed that the shouts of victory on the battlefield were from Octavius’s side.
Mistrust of good success hath done this deed.
As all these actions – that are based on hypothesis, are seen in the entire play I believe that the message that Shakespeare want to bring over to us is that we should not make decisions on mere assumptions.