Most get into politics for the right reasons. They want to do good.
Party politics mucks that up quickly with its ‘either you are with us or against us’ mentality. So you end up voting for things that you wouldn’t normally vote for, sell your soul to get votes for things of interest to you, and find that people that give you money didn’t actually ‘give’ it … they were just prepaying for favors to be asked later.
Lest we forget, the first job of a politician after winning an election is to start working on getting reelected. All else pales into insignificance.