Most members wanted to kill Caesar before he became too powerful. Not in every case it is specific. Some may have done it for personal and selfish reasons, others for selfless reasons. For example, Brutus did love Caesar, but loved Rome more, so he killed Caesar for the greater good of Rome. If he let Caesar rule, he would be a corrupt tyrant, and Brutus could not let that happen.
Shakespeare deliberately emphasized that the motivations of conspirators are highly variable. Most of the minor members of the cabal are moved by personal grudges. Cassius, by contrast, takes part in the conspiracy against Caesar owing chiefly to his envy of the lofty and undeserved stature accorded to Caesar as a demigod. Brutus, however, has no personal grudge against Caesar, nor does Brutus resent Caesar's stature in the eyes of the people. His principal concern is that Caesar will transform Rome from a quasi-Republic into a dictatorship. Although Julius Caesar is stymied in this regard, Roman history bore out Brutus's analysis.