In Julius Caesar, how does Brutus begin to justify joining the conspiracy against Caesar?

You have asked a question that goes to the heart of one of
the themes of the play - the power of rhetoric and manipulation. The speech you want to
examine very closely is Brutus' soliloquy in Act II scene 1, where Brutus tries to
persuade himself into being involved in the conspiracy. If you were directing this play
yourself, you need to think about whether Brutus is actually getting involved for the
noble motives he possesses, or whether this speech is actually ironic: in talking about
the danger of ambition and how this will probably lead Caesar to become despotic, Brutus
is blind to his own ambition and how the same dangers await
him.

Brutus starts off my saying there is no personal
reason for him to kill Caesar:

readability="9">

It must be by his death; and for my
part,

I know no personal cause to spurn at
him,

But for the
general.

The general good can
be the only motive for assassinating Caesar, Brutus assures himself, before moving on to
the crux of the issue:

He
would be crowned.

readability="16">

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.

Note here how Brutus
uses a metaphor of an adder to compare what might happen to Caesar if he is crowned.
Crowning Caesar would give him more power, giving him a "sting" that he could do serious
damage with to democracy.

Although Brutus acknowledges that
Caesar has shown himself to be worthy of the power he has received up to now, Brutus
falls back on aphorisms and "common proof" to persuade him of the danger were Caesar's
power to grow bigger:

readability="19">

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 teh clouds, scorning the base
degrees

By which he did ascend. So Caesar
may;

Brutus thus uses such
"common proof" to persuade him of the danger, though the use of words such as "may" of
course perhaps makes us think that Brutus' conclusion is by no means certain, thus
suggesting that other motives come into play that perhaps Brutus is blind to, such as
his own ambition and envy of Caesar. Either way, by the end of the speech, Brutus has
convinced himself:

readability="11">

And therefore think him as a serpent's
egg

Which hatched, would as his kind grow
mischievous,

And kill him in the
shell.

Brutus and his
conspirators must "kill" Caesar now before he "hatches" into the serpent that, according
to Brutus, he will obviously become due to the amount of power he is gaining. His choice
is made and Brutus has chosen his destiny.