Asklemmy

If you have the time, put some resumes out before accepting the first thing to come along. I don't know how things are in Germany, but I've always believed it's easier to find a job while you are still working. That said, if the new position, pay and work culture seem good, taking the position for now may be a good choice. You can always job hunt later.

As for how you conduct yourself, I'd always suggest conducting yourself in a professional manner. While you may have zero intention of coming back to this organization, you never know when you are going to run across the people you work with again. And the next time they may be in a position to help or hurt you. For example, I worked for a company really early in my career which started falling apart quickly. Towards the end of my time there, they announced they were closing the office I worked at and basically gave my department a big "fuck you". I could have gone out causing trouble or just worked my time until I left for greener pastures. I did the latter. Years latter, I was applying for a job I really wanted and an important member of the hiring team had worked with me at the first job. Not as my boss, just someone in another department. He remembered my work and work quality and had effectively said, "yup, hire this guy". While I have long since left that job as well, his confidence in me changed the trajectory of my career.

Maybe it's different over there, but I've always heard that "it's who you know, not what you know" that gets you hired. And I've run into that in my own career. You don't want to be a pushover, but keeping professional relationships professional can pay dividends down the line. Do the job you are paid for, don't make messes for other people and at least try to be professional in your dealings with others. You may be able to climb the ladder quickly today by being an asshole, but you never know if the fingers you step on today will be attached to the hand you will need to help you tomorrow.