Why does Brutus decide to assassinate Caesar in Julius Caesar by William Shakespeare?

Brutus is the primary character in Julius
Caesar by William Shakespeare. Cassius knows that he needs to have Brutus as
a member of the conspiracy.  He is popular with the Roman citizens and the other
senators as well. 

With his sensitivity and logical
thinking, Brutus is troubled by Caesar’s rise to power.  He has been a trusted friend of
Caesar’s; but now, Brutus has pulled away from everyone.  Cassius tells Brutus that his
friends have noticed that there is something wrong with how Brutus is acting.  Brutus
explains that he is at war within himself.

In Act I, Scene
ii, Cassius explains his feelings about Caesar to Brutus.  Cassius does like Caesar.  He
thinks that he is weak, no more worthy to be the emperor than is Cassius, and he has had
to save his life on two occasions.  This infuriates Cassius that now Caesar is like a
god to the people. 

Brutus tells Cassius that he will think
about everything that he has said with regard to Caesar being crowned.  Then, he asks
Cassius to come to his house at another time to discuss things
further.

Act II, Scene ii, begins with Brutus in his
garden.  He asks Lucius to tell him with the date is.  Lucius tells him that it is the
Ides of March.  It is obvious that Brutus has not
slept. 

Unlike Cassius, Brutus has nothing against Caesar
personally.  It is the possibility of Caesar becoming more powerful that worries Brutus.
Brutus uses the idea that the assassination of Caesar must be for the good of the Roman
citizenry. In a soliloquy, Brutus gives three analogies to explain why he believes that
Caesar should be killed. 

These  analogies explain Brutus’s
reasoning  for killing Caesar:

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It must be by his death, and, for my
part, 
I know no personal cause to spurn at him, 
But for the
general...

1. The
   first analogy that Brutus uses concerns a venomous snake that a person
   may come upon while walking. A man must avoid the serpent
   since the snake can be dangerous.  If the crowd wants Caesar to be king, then this
   might spur him to be crowned. As the snake given the
   opportunity to bite someone, Caesar may become too
   powerful. 


2. The second analogy replicates the scene of a
   man climbing the ladder of success. 
   As the man climbs, he needs the support
   of other people to help him up.  When attains the top and becomes successful, he forgets
   about those who gave him aide as he ascended.  Caesar may
   do this as well. 


3. The last example is a
   nest of serpent eggs.  The serpent inside the egg is harmless. It is not until he comes
   out of the egg that the snake then has the potential to sting a person. To prevent this
   from happening, kill the snake while it is the egg, he does not have the opportunity to
   hurt someone.  This is why Caesar should be assassinated.  Kill him like the serpent’s
   egg before he can become too
   powerful.

Unfortunately, Brutus has made an
unusual decision: Murder  Caesar based on possibilities, rather than on things that he
has actually done.  When Cassius, Brutus becomes a part of the assassination to kill
Caesar.