Brutus uses "we" when he refers to the fact Caesar has to be killed. He refers to this fact at the beginning and at the end of the soliloquy.
Killing Caesar will prevent him from becoming a tyrant.
This is what Brutus imagines. He thinks that power will turn Caesar into a tyrant. Brutus compares this potential ambition to the growth of a poisonous snake. He thinks "the snake" has to be killed while it is in the shell.
Brutus plans to kill Caesar. In fact it is Cassius who plans to kill Caesar and Brutus after evaluating Cassius' plan, will also plan to kill him.
The options are not right:
-Caesar is already a tyrant. In Brutus' opinion, he is not but he will become one if they do not kill him before he gets the crown.
-Brutus despises Caesar and wants to rule himself. Brutus does not hate Caesar. He respects and honours him but he loves Rome more than Caesar.
-Killing Caesar will prevent him from becoming a tyrant.
Brutus does not despite Caesar. In fact, he does not feel any grievance against him. Anyway, he is contamplating the option of participating in a plot to assassinate him because he believes that once in power the Caesar will become a tyrant.
