In Shakespeare's Julius Caesar, why does Brutus not want Caesar to be king?

In the first act of Julius Caesar,
Cassius persuades Brutus that Caesar has tyrannical tendencies.  For one thing, he has
defeated Pompey, who used to be his ally.  Then, he enters Rome with Marc Antony;
together they make a show of Antony's attempting to crown him with Caesar's refusing
this crown three times. Cassius tells Brutus that Caesar is a "colossus" standing over
them, and ends by saying that "honor is the subject of my story"
(1.2.92).

In the privacy of his garden in Act II, Brutus
ponders all that he has seen and heard this day.  After having received the forged
letters that are supposedly from senators who inveigh the ruler, Brutus tells
himself, 

"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
crowned.

How that might change his nature, there's the
question.

Th' abuse of greatness is when it
disjoins

Remorse from power; and, to speak truth of
Caesar,

I have not known when his affections
swayed

More than his reason....
(2.1.10-21)

 Brutus worries
that Caesar may rise to power and lose his humility and become the tyrannt that Cassius
describes :

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Looks in the clouds, scorning the base
degrees

By which he did ascend.  So Caesar
may;

Then lest he may,
prevent....

And, therefore, think him as a serpent's
egg,

When hatched, would as his kind grown
mischievous,

And kill him in the shell.
(2.1.26-34)

It is Brutus's
fear of Caesar's tyranny which leads him to join the conspirators and attempt the
assasination.  He nobly tells the Romans that he did love Caesar, but he loved Rome
more.