I mean Bush is so powerful, He killed his own 5000+ and millions around the world. So why would he have to lie ? Something seems fishy

Because he is a politician. Most politicians lie. It's part of their job.

why is it has to be a part of that job ?

Politician A speaks the truth. The truth is unpleasant. People do not like this truth or, therefore, what politician A is saying. The people do not vote for politician A, and so he does not get elected into a position of power.

Politician B tells the people what they want to hear, no matter how untrue it may be. The people like what he is saying. The people vote for politician B, and so he gets elected to a position of power.

The current flaws of the democratic system in the US and Britain, and elsewhere, allow these things to happen. Changes, it seems, must be made. Quite often of course, it isn't lies, it is promises that aren't kept.

Consider also that many politicians are corrupt, or unfair. Look at the headlines in the British news: John Prescott (the Deputy Prime Minister) has had an affair with his secretary, yet he isn't resigning. The government have said that this is a "private affair". David Blunkett (the former Home Secretary), who did the exact same thing only a few months ago, was pressured to resign and he did, yet that somehow wasn't a "private affair".

so what happens if the law is such that,

if a politician cant keep his promise he goes to jail for 1 year ?

Perhaps more, and lose his pension (politicians shouldn't be paid so much anyway). This would either increase the number of promises being met or increase the realism of promises, which may increase the effectiveness of the system as a whole.

Isn't the US effectively a two-party system anyway?