Brutus and Cassius die in rather simliar ways, but Caesar's death is very different. What is the meaning of this contrast?

This is a very good question, as one would imagine that
Shakespeare is making a point about the characters through how they die.  This could be
possible, and I'll look at the different deaths thematically in a moment, but first, it
is important to note that this play is based upon historical fact and is accurately
reporting the method of death for each real life character.  So, whether, hisotrically
speaking, Fate directed their deaths and provided "meaning" in the "contrast," is not
something we can know.

In the play, however, honorable
action is a key theme.  This theme plays into the different deaths in that both Brutus
and Cassius take their own lives (with assistance), while Caesar is set upon and
murdered.  Brutus and Cassius decide to end their lives, as they see this as the most
honorable course of action left to them.  Caesar is killed because others believe that
he is acting dishonorably, even though he, Caesar, sees himself as completely
honorable.

So, while the deaths are required by historical
fact, we can still find some meaning in contrasting the deaths of the of Cassius, Brutus
and Caesar in relation to the theme of honor.